Direct Cheat Detection
Websites can detect GM Cheems by looking for it specifically. Unfortunately, I'm not at liberty to discuss this, because I'm constantly working on ways to avoid detection and can't give out details.
Ban Decision Factor: High
If they detect you have it installed, that's usually a quick ban.
The only way to avoid this is to not install GM Cheems. Always use GM Cheems with alt accounts you're willing to lose. Always install it on a separate Chrome profile and only log in to your alt accounts on that profile.
You don't want to be in a situation where you have GM Cheems installed and you're logged in on an account you're not willing to lose, even if you don't cheat. Simply having it installed and being logged in to your account on the same Chrome profile will make you suspect.