Can I use Garchomp & Giratina GX's "GG End GX" attack to remove all of my opponent's Pokemon from play?
Yes, if your opponent only has 1 (or 2, with the extra energy) in play, you could remove those Pokemon. Then you win the game!
Source: Unified Minds FAQ (2019-07-19), TPCi Rules Team (2019-08-01)