Someone can be timing out for many reasons.
- Trying other game betas might help (x86-64 chromium, Dev branch, and Pre-release branch)
- Clearing download cache
- Verifying integrity of files
- GMOD/Steam re-install
- reduce the number of mods downloaded!
- locate the cl.db file located in the Garry's Mod folder and then delete it. Then load into the server. (This has been a fix so far)
- In some cases, a PC reset might be needed (This should only be decided as a last resort).
How to verify files?
https://help.steampowered.com/en/faqs/view/0C48-FCBD-DA71-93EB
How to clear download cache?
https://help.steampowered.com/en/faqs/view/6AD7-820D-8BE5-E51F