Most games, including, I think GTAV depend on the performance of a single master core.
I would pick the i3-6100 for a new build.
It has superior single thread performance.
If you primarily play multiplayer with lots of participants, then the higher total capability of a i5 might be better.
For most other types, 2-3 cores is the most they can effectively use.
For the best of both worlds, a I5-6600K with a good overclock is as good as it gets today.