Direct Cheat Detection
Websites can detect GM Cheems by looking for it specifically.
Unfortunately, I'm not at liberty to discuss this in detail, because I'm constantly working on ways to avoid detection.
Ban Decision Factor: High
If they detect you have it installed, your account will be flagged. You may get banned right away but not always.
Chess.com is known to have messaged users informing them that they detected an extension and that they'll give you chance to uninstall it, within a grace period.
Avoidances
Disable the overlay and only use the DevTools (requires Pro)
Websites can detect GM Cheems by looking for the elements in the overlay (i.e. sidebar, arrows, etc.).
To reduce chances of getting detected, go to the Streamproof options page and disable the GM Cheems overlay. Afterwards, only use the GM Cheems DevTools tab.
All features should continue to work as expected. More info here.
Always use the latest version of GM Cheems
I'm always pushing anti-cheat bypass improvements in each update and I don't always announce them.
Please make sure you're using the latest version of the extension at all times.