It is a bit difficult to really tell.
Games are written differently.
Some games are more processor dependent, while the majority is more GPU dependent.
The thing is however by your rig.
GTX1050 is still ok, not among the best but still ok.
Your I5 2400 is already quite old and already a noticeable bottlneck for e.g. 1070 on some new proc heavy AAA games.
Now back to which one you should upgrade first.
On GPU heavy games, going for GTX1060 or higher will do just fine, even with 1070.
On CPU/Processor bound games, going for better processor is better.
For the longer run, I would upgrade the processor first and the GPU later.
BTW, I would leave PUBG out of the topic. This game is badly optimized.
For Rainbow 6 and ARMA 3, you need to upgrade both proc and GPU to gain more enjoyable FPS but your I5 2400 should be the bigger limiting factor.
This is also another reason to go for the processor upgrade first.