i will tell everything in steps.
1.click gmod on steam
2.click workshop
3.click Browse then collections
4.then click create collection
5.name your collection and add image
6.when u scroll down u have categories, just choose addon in content type.
7.click subscribed
8.the hardest step is to add every addon u want , that can be hard bc if u have so many addons u must click them all.
9.click save and continue when u are done
10.click upload
11.then copy the link of collection and send it to your friends you want to play with on steam.
12.they must click subscribe all
13.enjoy