Problem with the PC market is, everything is a "it depends." The stuff people have in PC is wildly different, what you upgrade is entirely dependent on what you upgraded last and what you needed.
If you are new to PC gaming, and you like many normal people, probably just got an office computer cheap or you bought yours like more than 5 years ago, then maybe you should upgrade to a gaming monitor with high refresh rate if you want action, or something with better colour and detail if you are more into slow, immersive games. But if you already upgraded a monitor perhaps in the last couple years, and you are running a 1060, yeah, a GPU is probably where is at if you do want to upgrade. It is probably not wrong to say there is a bit of a disconnect between what monitor to have in relation to your GPU/CPU, and even among those two you have a better chance finding a recommendation from a random Reddit post than any tech outlet.
Honestly, if I am to make a out there recommendation on what to get, is probably better speakers or headphones. For my Playstation, my TV surround sound unit is having a little trouble recently, and therefore I had to use TV speakers; the difference is silly and I am using headphones even when gaming alone. Whereas on my desktop, after fiddling around for ages on cheap speakers and stuff like Steelseries and Logitech (basically "gamer" orientated junk), I opted for Q-Acoustics speakers, wired Audio Technica headphones that I semi-retired when I decided to use wireless, and a Blue Yeti mic (I had to do presentations on Zoom, so that's a plus). Together with a cheap DAC, I have a great sound set up for everything I do with my desktop. Thing is, save for the speaker, which I got because I wanted high quality output for music, the stuff I got wasn't expensive but the quality is top notch.