If you want to improve gaming performance of your pc, you need better GPU.
But for better GPU you need more capable PSU of gaming grade quality and depending on GPU choice also better CPU.
So upgrade order should be:
1. PSU
2. GPU
3. CPU.
SSD would not improve gaming by much, only boot times and game loading times if you put game on SSD.
You can get it first or last - no big difference.
In summary - better first get enough money to upgrade everything at the same time.