While looking at the Japan Championship decks, I noticed that only one Gardevoir-GX deck did well, but it was built in a weird way; there was only one copy of Rare Candy in the deck, and there weren't any copies of Tapu Lele-GX or Guzma. Instead, it focused on slowly evolving Ralts into Kirlia into Gardevoir with the help of Sylveon-GX, then spamming Acerola to deny Prize Cards (At least that's how I interpreted the decklist). Here is my take on the deck:
Pokemon (16)
4x Ralts (Burning Shadows)
4x Kirlia (Burning Shadows)
3x Gardevoir-GX (Burning Shadows)
3x Eevee (Ability one from Sun and Moon)
2x Sylveon-GX (Guardians Rising)
Trainers (30)
4x Cynthia
1x Tate and Liza
2x Judge
1x Lillie
4x Acerola
4x Nest Ball
4x Escape Board
1x Choice Band
1x Rare Candy
4x Acro Bike
2x Rescue Stretcher
1x Timer Ball
1x Pal Pad
Energy (14)
10x Fairy Energy
4x Double Colorless Energy
Pokemon (16)
4x Ralts (Burning Shadows)
4x Kirlia (Burning Shadows)
3x Gardevoir-GX (Burning Shadows)
3x Eevee (Ability one from Sun and Moon)
2x Sylveon-GX (Guardians Rising)
Trainers (30)
4x Cynthia
1x Tate and Liza
2x Judge
1x Lillie
4x Acerola
4x Nest Ball
4x Escape Board
1x Choice Band
1x Rare Candy
4x Acro Bike
2x Rescue Stretcher
1x Timer Ball
1x Pal Pad
Energy (14)
10x Fairy Energy
4x Double Colorless Energy