Why the mods doesn't appear in the addon menu in Gmod game?

I tried to install mods without Steam Workshop from Skymods and put it in the addons folder, but when i open the Gmod game and open the addon menu, it does not appear in subscribed, trending, top rated, latest uploads, friends uploads, and my uploads section addons. It might be because i just installed the game from websites like Igggames that is incompatible with the Steam app, or maybe because of mods that are not compatible with the Steam Workshop—i have to re-download it but with Steam app (through by purchasing) and download the mods there individually, or something like that so i couldn't get the mods in Gmod. Can anyone explain to this problem or fix it please?

Can anyone explain why my mods doesn't appear in the addons menu in Gmod?

Sorry for the necro-bump.. but I will answer your question:

Addons come in 2 flavors for garry's mod.

1. Legacy Addons (These are the addons you download and put in your addons folder)
2. Workshop Addons (These are the ones you subscribe to on Workshop)

The in-game addons menu only shows you Workshop subscriptions. Any legacy addons or gamemodes you download and install will not show up in this menu. It's a bit annoying, to be sure, but this is just how it works.