please post here your pc specifications and what what graphics settings are you using in the game. It is difficult for us to help you if we don't know your pc and how are you running the game.
Does this glitch happen only with this game? have you tried to patch it to the last version?
I found this, it should help you: https://support.steampowered.com/kb_...3803-DFGL-0112
"Half-Life 2 Video Error: Gray Fog Glitch:
- ATI Card Owners: Enable Catalyst AI
Right-click on your desktop and select Catalyst Control Panel
In the tree in the left window pane, select 3D > Catalyst AI
Uncheck Disable Catalyst AI. The slider may be left in either Basic or Advanced mode.
- nVIDIA Card Owners
If you see this error and you have a nVIDIA, please update your video card drivers
to the latest version. See the Updating Drivers topic for further instructions.
- Alternate solution
This solution may be necessary if you are seeing this problem with an Intel GMA 3 or 4 series integrated GPU, or in some games that Catalyst AI does not solve this issue for. While this unfortunately disables a number of features, such as color correction and High Dynamic Range (HDR) lighting, some users may prefer to sacrifice these features to correct this problem.
Open the game, then bring up the console.
Type the following command in the console, then press Enter: mat_dxlevel 81
Allow the game some time to restart the video engine (similar to the time taken to change video resolutions)."
Hope this will help you, good luck
If the glitch isn't solved in this way, let me know!