Hi there,
I am sorry that is happening for you.
First I want to say that Steam has a 2 hour window for you to refund the game, so if you played for more than 2 hours than they won't initiate a refund. I just wanted to let you know before you launch the game again in case we aren't able to solve this. I am not sure what itch.io's policy is for returns, but they may be more lenient.
Now as far as troubleshooting: screen flickering typically occurs due to incompatibility between your system and the game itself. I would need to know more about your PC specs. Garden of Seif can be more graphic heavy than other visual novels due to the cinematic scenes.
The issue could be your screen refresh rate. If the refresh rate isn't synchronized with your video card, It can cause a flickering image. https://www.coolblue.nl/en/advice/set-up-refresh-rate-monitor.html
This reddit post has some other recommendations:
https://www.reddit.com/r/visualnovels/comments/ypjas/so_any_of_you_have_problems...
Let me know if any of these help.