i know how to fix the hl2.exe error on steam source engine game on vista system
1. Right click on your game in steam
2. Click "properties"
3. Click "set launch options"
4. enter the cmd "-dxlevel 81" (without ")
5. click "ok" and close properties
6. launch the game
7. if the game is not working properly try the cmd "-dxlevel 70"
setting the direct x level worked for me
the fix can be found under steam troubleshooter under game crashes section allthough steam also tell you to type the cmds
and +mat_forcehardwaresync 0
and these can also fix some games problems
but it seems direct x is the only thing you need to change on gmod10
this is a link to the support page containing this fix https://support.steampowered.com/kb_...7169-TIAV-5361
scroll to Set Game Launch Options and Disable Problematic Hardware