If you have to reinstall you go to steam and right click the game and click "Delete local content". It'll delete the game and move it to the uninstalled list so you can install it again.
But before you do that, try going to the steam folder > steamapps>username>garrysmod and rename or delete the garrysmod folder inside the first garrysmod folder. When you start the game it'll make a new fresh one. That might fix your problems.
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