If the majority of people will not use the new 'client' = reach the consensus, then it will be a problem to everyone because two chain cannot exist forever.
I think there is even a remote possibility for both chains to coexist literally forever because both are supported by parts of the community. I don't see why Bitcoin core would die as long as it has support?
It won't die so easy.
Another thing: if Gainvchain makes technical problems and people go back to Bitcoin core those who have spent their btc on Gavincoin will have lost all their BTC on Core aswell.