Gyarados is still pretty common despite Xerneas tbolt. Also if people start using Zygarde then they can't run Xerneas too. Gyarados is totally fine but an ice fang is not OHKOing Zygarde even if it doesn't have yache berry regardless of expert belt or not.
252+ Atk Expert Belt Gyarados Ice Fang vs. 252 HP / 0 Def Zygarde: 114-137 (53 - 63.7%) -- guaranteed 2HKO
252+ Atk Expert Belt Gyarados Ice Fang+ vs. 252 HP / 0 Def Zygarde: 148-178 (68.8 - 82.7%) -- guaranteed 2HKO
I'd say Wacan is still the best option on Gyarados for random tbolts and tpunches.
Assuming Skarm is the mega and Chandelure is sash or life orb, you probably want something to hit the Garchomps. Nevermeltice Glaceon, Vaporeon or AV Clawitzer possibly. Alternatively something like Rocky Helmet Hippo with curse keeps Garchomp at bay while also being immune to charge tbolt shenanigans from ampharos.