It's a browser extension - a bit of code that runs some scripts, in this case modifying the style of the page when you go to a specific domain (gmail). Extensions are run inside of the security sandbox for that domain with a higher privilege level than normal webpages - consequently, the extension could send all your email and activity from other tabs to a 3rd party. The Gmelius website says they don't though. You could, if you were so inclined, check the code inside the extension package if you wanted. If it was doing anything dodgy they'd be found out pretty quickly.

tl;dr Yes, it could read all your email. I doubt it does though.