Validate the game:

From the Library section, right-click on the game and select Properties from the menu.
Select the Local files tab and click the Verify integrity of game files... button.
Steam will verify the game's files - this process may take several minutes..

Or you can choose to not download any custom files:
1.Go to options in garry's mod
2. Go to multiplayer
3. You will see down below a option button with the one that says: "do not download any custom files"
Hope this worked for you!