Welcome to the troubleshooter, this is probably where you'll find the solution to your problem.
Please read this all before submitting a ticket. You'll be able to submit a ticket when you reach the bottom of the page.
Do NOT skip over anything! All of this information is important and will save you time :D
Can't sign into Steam through XEON?

You need to switch to the "chromium" branch on Garry's Mod. Right click Garry's Mod on Steam and then click "Betas."