ElGamal signature - exploiting fallacious implementation
As Wikipedia’s Chinese remainder theorem article states:


  The Chinese remainder theorem is a theorem of number theory, which states that if one knows the remainders of the Euclidean division of an integer n by several integers, then one can determine uniquely the remainder of the division of n by the product of these integers, under the condition that the divisors are pairwise coprime.


Using that, you can solve this by finding a $\gamma$ satisfying 

$$\gamma \equiv 0 \pmod{p-1}$$

and 

$$\gamma \equiv \alpha^x \pmod{p}$$ 

and you’re done.