Depends on the resolution you want to game at. The maximum FPS you can get is mostly determined by the CPU (using reduced settings as needed). The GPU will determine the FPS you can get once below that threshold. So if you got a GTX 1080ti, for example, for 720p, even at the highest settings, it'll be barely used, because the CPU holds it back, but at 4K, the GTX 1080ti will be the component holding back performance as long as you use near maxed settings. If you play at 4K, but at low settings (I've seen people trying this), you'll be CPU bound.
How you play and the resolution you use means a lot. More than the CPU does in most cases.