Gamma correction isn't "Addition" its exponentiation (i.e. raise to the power for the non-mathmatical).

The function you want is "Pow" and the following node inputs/values :-
GC: Input1 <- the colour you want to correct.
    Input2 <- 2.2
AntiGC: Input1 <- the colour you want to correct.
        Input2 <- 1 / 2.2 (use a Math_Function node set to "Divide" with Input2 set to 2.2)

Make sure that you use "Color_Math" nodes for (Anti)GC calculations on colours ("Math_Function" nodes will change to greyscale, which probably isn't what you want).