Purchase a product key and activate Windows down the line. You should be good for now even without it. Wont affect gameplay... https://www.howtogeek.com/244678/you-dont-need-a-product-key-to-install-and-use-windows-10/
GTA5 seems to have issues with VSYNC in my experience. For example, on a 60Hz monitor with VSYNC ON if I drop below 60FPS I get a stutter/judder that is really annoying.
I fixed this by forcing "Adaptive VSYNC" then tweaking the game settings so I don't drop below 60FPS much. When I do drop I get some SCREEN TEARING instead of the stutter/judder effect I got before.
Here's how for NVidia: (start and close GTA5 first)
NVidia Control Panel-> manage 3d settings-> add game -> ... (enable Adaptive VSYNC)-> SAVE
You have a few options you should tweak based on (assume 60Hz monitor):
1) VSYNC ON (bad with drops below 60FPS)
2) VSYNC OFF (screen tearing can be very annoying... if not aim for 40FPS average... actually GEFORCE EXPERIENCE setting if applied for GTA5 is I believe 40FPS VSYNC OFF and it auto-tweaks the settings for you)
3) Adaptive VSYNC (60FPS VSYNC ON, but disables VSYNC if you can't output 60FPS... so screen tear but not added stutter below 60FPS)
4) "Half Refresh" version of Adaptive VSYNC. (caps at 30FPS VSYNC ON... more sluggish than Adaptive VSYNC but allows higher quality)