Ownership in RPC function change?

if i run this code below on client side,

void Foo();

void Foo_Implementation(float value)
     _gameState->_someVariable = value; 

is that _gameState in function on the Server Side?

and Next time when i run this code on the Server side, is that _someVariable of _gameState already changed?

In your example you forgot to add the float value to the Foo function declaration too.

Yes the Foo_Implementation code is run on the Server if you call Foo from an Owning Client.