Crash without crash log.

[Ass. Dir. Elias Shaw, PhDeez]

If I load into a map, the game crashes after about 2 seconds, sometimes sooner. I have no idea why, since I can't find a crash log anywhere within the files. It does this on every map.

[Fearagen]

Verify Integrity of your local game files.

Right click Garry's Mod in your Library >> Properties >> Local Files >> Verify Integrity.
Sounds like something corrupted and this should repair the file(s).
If that does not work then do the following: Check your mods to make sure you don't have anything that would cause such an issue. If you do not have a single mod that's being unstable/glitchy then uninstall gmod via the steam library, go to C:\Program Files (x86)\Steam\steamapps\common and delete the "GarrysMod" folder. (Steam usually does not remove game files from this folder after said game(s) was uninstalled). and then reinstall gmod, wait for all your mods to download from the workshop after launching, test a map to see if the issue somehow is will present.

[Temar]

I have had the same problem but fixed it recently. Try doing this: Try the x64-86 Beta branch for Garry's Mod in Steam.
To change Beta branches for Garry's Mod, do the following:
Open your Steam Library
Right click on Garry's Mod and select "Properties"
Switch to the "Betas" tab
Select the preferred x64-86 Beta version from the dropdown list