Looking for where addons are downloaded in files

You can find the GMA (Garry's Mod Addon) located in a few places:

• addons on the Server and are named similar to ds_x.gma
  • garrysmod\cache\srcds may also have some named x.gma

You're probably looking for:

• garrysmod\downloads\server on the client side and are named x.gma

Keep in mind if you're looking for certain addon that x is the steam workshop addon ID found after the ID parameter for example: steamcommunity.com/sharedfiles/filedetails/?id=123123 is 123123.gma

Also, sometimes the files will be compressed and you'll need to extract them using an archive program like 7 Zip.

If you want to you can download the addons from a site, I suggest: http://steamworkshopdownloader.io/

If you already have the files you can try GMoshUI: https://github.com/FPtje/gmosh (this also helps to publish addons to the Steam)

Give this Steam community guide a shot. I would recommend visiting the Steam Community forums as the best resource on topics such as modding, support and discussion on apps like Garry's Mod. There are alternatives out there, but you'll likely have the best results at the Steam Community. Since Steam is the primary source for support, finding a decent alternative may be difficult. Hope the guide is what you're looking for. Your question is a little unclear, so I hope my answer is not too out-of-bounds for what you need. If this answer doesn't resolve your issue, try refining your question with more details.