Uh oh. Seems like we didn't get Lost World yet - so Gigas will thriumph with Triumphant.
While everyone probably knows this already, pretty much all new trainers/supporters should be very Gigas-compatible. However, they will also be pretty Gyarados-compatible so watching that matchup is always not a bad idea.
Things to consider:
Twins - What can I say. The perfect combo with Sacrifice is here. No need to blow up stupid Electrodes to get behind in prizes - you'll have all the benefits of Sacrifice and more with Twins. This turbo-boots Gigas so well it isn't even funny. The idea here is to start Sacrificing, then Twins, then either Drag off important stuff while reusing Twins (VS Seeker) and setting up everything you ever have needed and more, or go for aggro Gigas blastering everything in your way. I tend to go with slower approach for certainity as Gigas only has limited amount of recovery capacity when one goes down, but blastering everything is certainly a worthwhile option for winning games quickly.
Junk Arm - I know me and Cetra may share a bit mixed opinions about this card, but IMO it's a card Gigas should play 4-of. And VS seeker. VS seeker is more of a card to get to your discard pile as early as possible in the game. Then, when you get Judge'd, Initiative'd or otherwise disrupted, Junk arm really extends your supporter availability with 4 (+ VS seeker, so 5). Of course, along VS seeker and all the supporters, it can reuse Communication, Rescue (=all the pokémon in discard pile), Warp Point/Switch, SSU, crazy tech trainers like Pokéturn and the list goes on. And the best part is - you don't have to Regice that often anymore, which has always been a painful card to play down in certain matchups. I've been ready to even drop Regice after adding Junk Arm to my deck, but kept it in for further testing for some things like Spiritombs and Dialga G (and of course other trainerlockers).
Black belt - a seemingly noobish card, but can serve an important purpose in the all-important battle against Promocroaks in SP decks. Also serves purpose when taking down belted Gyaradoses. This too should be made accessible with Junk Arm + VS Seeker combo, as it's the most flexible way for Gigas supporters (aside from one other card which I'll be returning to later).
Seeker - my least favourite from the pack, but an obvious include anyway. Reusing Mesprits without need to flip or sacrifice certainly isn't bad. It just takes a supporter slot where Gigas obviously has loads of better things to do as you can see above. Still, a powerlock enhancement, and a card that usually proves it's value in long, resource-intensive games. Also reusable with VS Seeker + Junk arm, possibly extending the powerlock with 6 turns in total. And it's fun to announce VS Seeker for Seeker.
Now, when all the best cards in the set are compatible with a certain deck, it usually rises in popularity. However, a certain other deck can pretty much use everything Gigas gets and more - maybe less Twins and Black Belt, but certainly more Rescue Energy.
Gyarados.
Gigas has always had a so-so match against the thing, and struggled with OHKOs. Now with Junk arm speeding up the action and Seeker restoring Gyarados to full health (and getting Warp/Rescue energies back, too) after every desperate blow that's thrown to it, where should one focus on? With Rescue energy, Judge/Giratina doesn't even get rid of the thing when it's set up, so there's a need for a new weak point for Gyarados to exploit. Would it be Giratina'ing after every Impersonate and trying to overpower from the start? Would it be reusing Warp points, trying to take cheap prizes from bench and constantly charging up new Gigases to compete in a prize battle? Or would you try some crazy techs like Relicanth's amnesia with hand control units to keep Gyarados unable to draw or attack?
Teching
Staraptor FB Lv.X in Gigas has been my little idea for a while now. Sure, it doesn't win you a Machamp matchup (other than Black belting Uxie Lv.Xs), or doesn't compete against Gengar (other than getting you the stuff you need besides the trainerlock), but it increases the overall functionality with the new supporter options. It also has a nifty anti-Gyarados attack which discards the stadium. Broken Time-Space is pretty much still the weak point of Gyarados, and if correctly executed, can freeze the deck or make your opponent to play pre-emptively (like getting extra Magikarps to the bench) in order to prevent the Mesprit + Giratina + Cyrus's Initiative + Defog knockout, which is probably the only thing that can stop Gyarados in it's tracks with all the draw and reusage it has going on. And when you succeed in taking out Broken Time-Space, just drag off the Magikarps. Staraptor also allows better teching of the supporters - 1 of every techy supporter is enough with Staraptor and VS Seeker-Junk Arm. Also, brutalizes SP decks without Luxray GL Lv.X as the all-mighty Toxicroak G Promo only does puny 40 + poison which is free-retreated off.
Dialga G Lv.X is also a hot contender for the tech spots here with it's fighting changes against LunaSol combo, Vileplume, Gengar Prime (which will fortunately not see too much play yet) and M2X. Doesn't fix Machamp and Donphan, though.
Abomasnow is still a contender for the tech spot. Now, with the supposed rise of the spread with LunaSol (which Gigas X can't touch), Aboma can return to the spread and turn the tides against some certain decks. Works wonders against Donphan, has some problems against Machamp Prime but functions well against regular one, can take out M2X and might even protect you from Thunder fall coming from ERL. A good option.
Drifblim UL is a pure Machamp-counter, which has a wall-negating opportunies too. Gigas itself is a wall-breaker though, so not so much needed. Can trick Gyarados and mirror match to your favour. Psychic energy requirement - not as fun.
If you're feeling Engineer's is still a must part of your Gigas (which I've dropped for good in favour of Sage's training, even more so with Junk arm's capabilities),
Bronzong SF is still available. Happens to have a good typing, and with Let loose, can 90 + Confuse your opponent's Machamp. Also fetches the correct basic energies if you opt for more special ones like DCE, Warp and stuff.
Double Smeargle is FUN. Smeargle itself is an awesome sacrifice card - when you get a special condition on your Gigas, you usually have to retreat and sacrifice anything you bring to the active position. Smeargle offers you some function for that, too. Double Smeargle can also be used for countering those Seeker-abusers, forcing them to take up a good chunk of cards while you Switch/Warp point your Smeargles, then drop the bomb with Giratina or if you get far enough, just take the last active out. Smeargle is also the most favourite starter in a Gigas deck.
Relicanth has lost a bit of it's power with absence of Roseanne's as it's not as easy to get Relicanth and Fighting energy to the same hand, but when against a mirror match, it gives you muchos lols to OHKO Gigas out of nowhere with Belted Relicanth (assuming 1 Expert Belt and 1 Q being attached, and with possibility for Crobat Drop or other means of 10 damage). Also works as your primary sniping machine in matchups where you can Drag off something heavy (read: Regice).
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Donphan Prime is your mirror device of choice. Utilizes the same energies as already in the deck, this can prove as decent Luxray counter, mirror counter and Mewtwo counter all in the same time. Donphan is also playable as it's own deck, so it tells a bit about the quality of the card.
Jirachi RR is a nice one-off setup fodder. Free retreat, brutal combos with things like Twins and Cyrus's Initiative and it's almighty single-twin power, this is a card that keeps rolling in my Gigas builds. If you don't feel like going through everything and more with Sableye, Jirachi is a lighter option. Cyrus2 combo can win games when charging more Gigases for your arsenal - double initiative can pretty much stop any deck in it's tracks in the format without constant draw.
Sableye can also be used and I'm liking it, as the first turn(s) with Gigas are usually spent by doing nothing too important anyway.
Chatot G is an underrated control card in an environment that just wishes control cards to be used. While Sacrifice prevents you from achieving a full control, a well-timed Chatot can be as good as hand full of Mesprits. Giga Blastering away the important topdecks or stalling just a couple more turns, even extending to getting you the Expert belt you need, this card has everything to support Gigas in it's blastering activities.
Sage's training is my engine of choice. While Engineer gets to keep all the cards, it makes the deck so much more vulnerable in the beginning of the game, where Gigas is yet to achieve the steamroll phase. You will not want to sacrifice your precious DCE to be able to maybe draw into a second energy - or even worse, you will start with a handful of engineers but no energy to go with. Sage discards can be handled with Junk Arms much more efficiently than before, although Sage wasn't even bad without the Junk arm. This makes Gigas's deckburning abilities out of this world, so be careful - although you can usually find a way to prevent decking before taking the 6 prizes. Giratina and Judge are good ways to go.
Well, that's the considerations from me with the Triumphant arriving. Happy toggling with your builds and hope to see some success in your side of an ocean, too! (Gigas has won 2 of 4 BRs and has 5 T2 placings here in Finland already this year, none of which were played by me
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