Upgrade Cpu or Gpu

I would differently upgrade the CPU. You have a decent GPU, but I would start CPU first. When playing games, depending on the magnitude of the graphics, you need both good CPU and GPU to process the graphics data to create flawless viewing on the monitor. If one is too overwhelming and the other is falling behind, you'll see some degraded gaming performance.

Your CPU is LGA 1151, but be sure to check what your motherboard socket supports I assume it is Z3##, ## is something what you need to figure out and what CPU is supported by your motherboard socket.

As @Sye_The-Vie said - I'd also recommend upgrading CPU. You can always sell your CPU + Mobo, and get used set, for eg. AMD Ryzen 5 3600 or similar (which you can upgrade up to 5000 series later).

If I'm using computer for gaming, I should definitely spend more on the GPU. Some games required both a smart CPU and a powerful GPU. Practicality , GPU need to upgrade first if your in a tight budget, then just follow the CPU afterwards.