shift tab is not working

Yes same with title shift tab is not working checked all option and reinstalled but not working

Using vista 32

When Steam updates, sometimes the game overlay needs to be re-added to your firewall. Check its settings, and make sure “Steam Game Overlay” is allowed.

Just in case the above did not solve your problem:
This is a steam issue, not a Gmod issue. You should ask this on the steam support site: http://support.steampowered.com