You are trying to find functions that satisfy the relationship: g(x)*g(y) = g(x+y) + x*y
Pretend I defined the function as g(z) = z + 1.
When z = x we have g(x) = x+1
When z = y we have g(y) = y + 1
When z = x+y we have g(x+y) = x + y + 1
So this definition of the function g satisfies the specified criterion.
I suspect its the only one, but I haven't attempted to prove it!