Do not switch a heavily modified Garry's Mod installation to 64-bit and troubleshoot everything at once. Preserve the current setup, test a clean launch, then restore addons in groups. That process separates a branch problem from one outdated module.

Record the working setup

Save the current launch options, active collections and server branch before changing anything. Back up custom configuration files that are not managed by Steam. A short record is enough to make rollback deliberate instead of guesswork.

A Garry's Mod scene with multiple addons
Large addon collections should be restored in groups after a clean 64-bit launch.Facepunch / Steam

Start with a clean local map

Select the 64-bit option in Steam and open a standard map without joining a server. Confirm that menus, input, saves and basic tools work. If the clean client fails, verify the game files before reintroducing workshop content.

Add workshop collections in groups

Restore maps and ordinary Lua addons first. Add binary modules and old tools last because architecture-specific code is the most likely source of a hard failure. Launch after each group. When a problem appears, split that group in half until the responsible addon is isolated.

Garry's Mod official artwork
Client and server branches should match before addon faults are diagnosed.Facepunch / Steam

Match the server

A local client can work while a dedicated server fails because the server has its own modules and startup command. Update both, confirm the selected architecture and read the first error in the server log rather than the cascade that follows it.

Roll back when the module matters more

The 64-bit option is useful, not mandatory. If an essential addon has no compatible binary, return to the working branch and report the module name, version and error to its maintainer. More addressable memory does not compensate for losing the tool the server was built around.