Everyone has been frustrated at some point for not being able to ALT+TAB Garry's Mod while it's loading a map or mounting workshop.
So here's a very simple fix for that you must know.

STEP 1 - While Garry's Mod is loading/stuck, hold WINDOWS + TAB. (Do not release this key combination yet)
STEP 2 - At the top you will see + Add new Desktop, click it.
STEP 3 - Now a new desktop has been created, simply drag your Garry's Mod instance to that new desktop.
STEP 4 - You can now use your computer on the (1) Desktop. To head back to Garry's Mod simply repeat Step 1 and switch to the (2) Desktop or drag it back to (1) Desktop.

Enjoy!