Wow, I thought the final structure looked too strange to be the right one!
The gamma decision was based on this thought process:
1. If the double bond was alpha, the cyclopropanone I will be cleaved to give a carboxylate in the presence of hydroxide - given the Clemmensen step, I wanted to keep the ketone intact.
2. If the double bond was beta, then it would probably isomerise in the presence of base
(first step) to the alpha alkene, which would be in conjugation with the carbonyl - more stable. This then brings up the problem in 1.
3. The only other option is gamma.