There isn't much synergy in Ho-Oh-GX's attack costs, which suggests it's meant purely for Eternal Light GX. At the moment, I can't think of a broken Stage 1 or 2 [R] Pokémon-EX or Pokémon-GX to use this with (only M Charizard-EX, M Houndoom-EX and Incineroar-GX spring to mind).
With more [R] support and better [R] Pokémon this gets much better though. I'll be on the lookout for Charizard-GX's compatibility with Ho-Oh-GX.
Necrozma-GX is cool; straight off the bat it blocks Tauros-GX, Drampa-GX and M Rayquaza-EX. However, without a spammable attack (or means of making it spammable), I'm not sure how good it will be.