It is always worth doing a compete delete of your gmod and reinstall. I would highly recommend following this guide step by step and it usually fixes these issues
STEP 3: GMOD Reinstall
In late January 2020, GMod released an update that changed the way that workshop and addon downloads are handled. If you have not done so since then I would HIGHLY *HIGHLY* recommend that you reinstall your full gmod to ensure that there are no conflicts.
Now this does not just mean going into steam and hitting uninstall and reinstall.
You will need to manually delete some files to ensure you have the freshest install.
First, uninstall in steam like you would normally do.
Then, go into your program files and locate the Garry's Mod folder. Mine looks like this D:\steamapps\steamapps\common\GarrysMod
Delete that whole folder.
Next, navigate to your workshop downloads folder which is now saved in a new location. Here is what mine looks like D:\steamapps\steamapps\workshop\content\4000
Delete that whole folder as well.
Now you are all set to reinstall through steam and can start back at step 1 if you are missing content.