You only need a new CPU if you are running into performance issues with what you already have. If you are generally happy with what you have, you can simply wait until better upgrade options become available or cheaper.
If you are running into performance issues in games, you could use a GPU load monitor to see how close to maxing out your GPU you are when that happens. If both the CPU and GPU "bottleneck" about as much, then upgrading one without the other may not improve things by much. If you choose to upgrade only one component from there, at least you will not be shocked that your upgrade failed to yield the expected benefits.