Sorry for the long wait, Here are a few solutions I can offer you:
1.If you're not on the chromium branch (64x) you can switch by right clicking on Garry's Mod, Properties, BETAS, you'll see 5 options, x86-64 is the one I used to use which is basically just 1 update after the 2nd option (chromium), they're basically the same.
2.Verify your files, You can do so by right clicking on Garry's Mod, Properties, Local Files & "click on Verify integrity of game files..."
lmk if it works after you've tried both solutions.
Out of curiosity though, what are your PC specs?