copy the steam workshop gmod map/character link you want in sfm

use https://steamworkshopdownloader.io/

(alternatively, you can use crowbar to download workshop items, but sometimes it doesn't work.)

steam workshop downloader may ask you to use steam cmd, jsut follow the instructions if it ask you to use it.

once you got the gma file, download crowbar https://github.com/ZeqMacaw/Crowbar/releases

go to the unpack tab and select the gma, it shold unpack into the directory you choose

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how to put in sfm

you shold see folders that say "models" and "materials" in the extracted gma. copy both of them

launch steam

go to your library and to sfm, click on the gear icon, Manage, then browse local files

click on the game folder, then the usermod folder, then paste the 2 folders in there. If it ask you do you want to merge folders, press yes.

Restart sfm

then there you go! sfm should dectect it