Is there a card similar to Lugia from Fates Collide that targets GXs instead of EXs?
A very similar card named Golisopod (9/145) was released in Guardians Rising expansion (released 5th May 2017), later reprinted with the same text as a promo (SM52) in Crimson Invasion Single Pack Blisters.


   &nbsp; &nbsp; Resolute Claws &nbsp; &nbsp; 80+
  If your opponent's Active Pokémon is a Pokémon-GX or a Pokémon-EX, this attack does 70 more damage (before applying Weakness and Resistance).


The card has similar, but better ability than your Lugia (Golisopod takes 30 less damage instead of Lugia's 20 and also Golisopod's Ability works on Bench too). The attack requires , making it hard to use in water deck. The bonus effect of the attack works against both Pokémon-GX and Pokémon-EX, making it the card you are looking for. Both base and bonus damages are higher than Lugia's ones (60+60 vs 80+70). 



There is currently no other card that makes bonus damage to Pokémon-GX.