Playing Gyarados, I managed to beat a Lostgar with Vileplume whenever I got a T2 Gyarados hitting for at least 60, which is quite a bit easier than you may think.
If I didn´t, they won.
This means Gyarados stands a chance if it can dish out the damage - the Lostgar player did Lost Zone one Magikarp with Devoluter, but after that they weren´t able to hurl too much whereas I took a few prizes here and there.
Since Lostgar plays on time, we had a timer running - Lostgar simply had issues getting Pokémon in the Lost Zone.
The reason for this is that I never had any Pokémon in my hand, meaning they had to Cursed Drop their way out - and not only did I play Seeker, they did.
In the end, the Lostgar player had 4 Pokémon in the Lost Zone because of three Seeker and that one Devoluter.
I´m positive Speed Gyarados is a good play for States, but I´m yet to see a list for it meaning I have virtually no idea about how to make it consistent and how to modify it for my personal playstyle (seriously, I´m horrible at building lists, but adjusting them is easier
)
My current build runs a normal balance with Trainers and Supporters alike, playing Poké Drawer+ and actually Sage´s Training. It may not be the best one though, so I´m looking into it.
As for teches, I would do Dialga G LV.X if I only had the space. Shutting off Vileplume, if only temporarily, helps a bunch in two matchups which otherwise are technically the worst for the deck.
Mew Prime I haven´t tested, but it seems quite risky.