Just my opinions: Neither if you are playing at 1080p 60hz, GPU if you are playing at 1440p+ 60hz, both if you are playing 1440p+ 120hz+
If you are going for the 100hz/120hz/160hz I would avoid ryzen, if you are going a standard 60hz then I think they would be worth checking out. But then you need a new mobo too. and more ram, possibly faster speeds depending on what yours are.
edit: at 1080p 60hz there still may be some games that could benefit moderately from a multi-threaded CPU, so as an afterthought if you absolutely wanted to upgrade, I would go with CPU in that case.