Seriously, guys, TEST it. If you have 1 Cresellia lv.x on your bench and an active Wailord with Leftovers, Holon WP and water (which is not hard to achieve - 2 stage 1s, tool and 2 energies), you will heal 40-50 damage every turn. Now GG can Psychic Lock you which would mean you cant use Cress x but you still heal 30-40 which means they will take at least 6 turns to KO you (without DRE). Or they can use Psychic Cut - either flip everything leaving wailord with 20 HP (of course no one is going to actually do that) or slowly cutting away. If they averaged 80 damage per attack, it will still take them at least 5 turns to KO you. Your Wailord (with help of Cress X) 2HKOs everything in GG. Now tell me a possible way GG can win when Wailord is set up.
And that is not mentioning Super Scoop Up, Xatu to transfer sleep and Potions (which are not as bad an idea as they may sound). Test the matchup please.
I would be much more worried about random Lighting Rogues than GG with Wailord.
When it comes to healing, based on your circumstances, you heal 10 each turn from Leftovers, 10 from Creselia LX, and 10 every other turn due to its body, or 5 every turn. Added together, you're healing 25 damage every turn, on average. Where are you getting 40-50?
Using Cresselia farces you to put it active, thus taking up another turn for energy. You could, use Moonlight stadium or Pheobe's Stadium, but that helps your opponent's G&G so much, by letting them hit and run.
Leftovers is complete crap, as playing just asks to take 80 damage from the most commonly used basic in the game, with one energy. Pachirisu for the win.
How do you expect to draw into your cards early on? Luck? You'll need a starting pokemon, reducing your energy supply to Wailord by one. The earliest you could attack would be turn 4, and only doing 50 damage each attack.
With such low damage output, it gives Magmortar enough time to get scrambles and Firestarted energies to do 200 damage in a single attack. If you honestly think having 2 Scrambles and 4 Fire energy on Magmortar is difficult to pull off, I suggest you read what Claydol's Pokepower does. Against G&G, the time it takes to take one single prize is the equivalent of having 2-4 Stage two's powered up, with DRE's and Srambles. Getting rushed with such numbers is impossible for a deck that can do nothing but match the damage given to it.
Also, I HAVE tested it, it was one of the first decks I tried when GE came out, but as I said, the deck just doesn't work in our metagame right now.