You may be confused. The gamma function is unrelated to the function x^y. Perhaps you are thinking of the relationship Gamma(n+1) = n! for non-negative integers n. Moreover, Gamma(0) is undefined (as is, more generally, Gamma(n) for non-positive integers n).

(For those who are curious, the gamma function is most easily defined as the Gamma(x) = integral from 0 to infinity of e^(-t) * t ^ (x-1) dt. Google can pretty easily be convinced to take you to more information about it. But it's pretty much unrelated to the topic at hand.)