Try the following steps:
1. Unsubscribe from the addon
2. Launch GMod and press Shift+Esc to open the console
3. Enter the following command: menu_cleanupgmas
4. Resubscribe to the addon

@Blueberry_pie
The 3rd Step Kinda Sounds Like
CleanUp PJs (Pajamas)

Thanks I got it working!

@blueberry_pie i tried what you sayed and nothing happens (sorry if i entered here without permission)

Originally posted by Rafix:

@blueberry_pie i tried what you sayed and nothing happens (sorry if i entered here without permission)

If those steps don't help, my only suggestion is to uninstall GMod entirely (delete the GarrysMod directory from your Steam/steamapps/common/ directory too) and reinstall it. However, this may cause you to lose some saved data and stuff that you've downloaded outside of the workshop, so do this at your own risk.

If it still doesn't work after that, there's nothing I can do. You may have more luck on the general GMod support forum, or you can open a support ticket[support.facepunchstudios.com].

Look what I found in console
Not loading addon 'D:\Games\Steam\steamapps\workshop\content\4000\1572373847/gm_boreas.gma' - addon header invalid
Couldn't mount addon file 'D:\Games\Steam\steamapps\workshop\content\4000\1572373847/gm_boreas.gma' from 'gm_boreas' (1572373847)

To solve that problem I downloaded map from Internet