First of all, there is a OU in double battles, VGC is a tier with its own Pokémons that this year is the open pokedex excluding box art legendaries and mythical Pokémons.
Now the sets:
Ninetales is amazingly fast so giving it a modest nature is strongly discouraged, better going timid, move wise i would change dazzling gleam for freezd-dry so you'll be able to hit 4X ludico, gastrodon, swampert etc
Kartana has a good set, I'd change one between smart strike or sacred sword for protect, if you choose to replace sacred sword change the item in grassium or focus sash (recommended)
Gastrodon : i don't know what those EV do but I'll run some calcs to survive some notable threats, I like that set especially toxic
Tapu fini: i don't like fini in this team, it's another water type and nullify gastrodon's toxic, if you want a tapu you should use tapu Koko because most of your team is really slow and you don't have sleep powder on venusaur, I suggest a standard set, tbolt, dazzling gleam, volt switch, protect 4ps/252sp.atk/252 speed timid life orb.
Garchomp is kinda ok, you'll need to swap stealth rock for protect, stealth rock is a terrible move in VGC, protect have to find its place in every moveset when it's possible, I'll change life orb for groundium but with eq having a way to hit a single target with a ground type move is often useful.
Venusaur : again, I don't know what those EV do but since you have Ninetales synthesis is an orribile move, swap it for protect
It seems you moved a OU team in VGC but it's not gonna work, they are very different playstyles, hope I helped :)
