I had this issue and couldn't resolve it myself and actually called a admin sit and was met with the Founder named Thy he helped me with communism...... I mean my errors and fixed them in a short time and I will explain the resolution for anything having similar issues.
Step 1. Unsubscribe from all current addons (You may Unsubscribe or uninstall)
Step 2. Goto this directory Program Files (x86)> Steam > Steamapps > Common > Garry's mod > Garry's mod > addons
Step 3. Remove every addon file in that location.
Step 4. Go to your steam click on Library
Step 5. Right click on Garry's Mod and goto property
Step 6. Goto local files and Verify Integrity of Game Files.
Step 7. After that's done re-launch Garry's mod and the server and reinstall the server addons if you need to
This should fix your issue, For some reason I still had some errors with cars after I did this but I slept a day and came back and the errors had resolved themselves.
Step 1. Unsubscribe from all current addons (You may Unsubscribe or uninstall)
Step 2. Goto this directory Program Files (x86)> Steam > Steamapps > Common > Garry's mod > Garry's mod > addons
Step 3. Remove every addon file in that location.
Step 4. Go to your steam click on Library
Step 5. Right click on Garry's Mod and goto property
Step 6. Goto local files and Verify Integrity of Game Files.
Step 7. After that's done re-launch Garry's mod and the server and reinstall the server addons if you need to
This should fix your issue, For some reason I still had some errors with cars after I did this but I slept a day and came back and the errors had resolved themselves.
