Which is a better CPU for gaming ?

Solution
The 3770 is better when both are clocked at stock speeds. If you factor in overclocking on the 3570k, then the overclocked i5 will win out on older titles that don't scale well across lots of cores/threads. For newer games, it would probably wind up being a wash between an overclocked 3570k and a 3770 due to newer titles benefiting from the additional threads the i7 offers.
The 3770 is better when both are clocked at stock speeds. If you factor in overclocking on the 3570k, then the overclocked i5 will win out on older titles that don't scale well across lots of cores/threads. For newer games, it would probably wind up being a wash between an overclocked 3570k and a 3770 due to newer titles benefiting from the additional threads the i7 offers.
 
Solution