If they are on the workshop, the easiest way ( and probably the quickest ) is to add them using workshop collections.
Create a collection on the workshop:
Add all the player models, maps, sweps etc you want on your server. Then, in your server start file ( .bat/.sh ) add:
+host_workshop_collection [collection_id] -authkey [your_auth_key]
Auth key can be got from here ( keep this private ):
Collection ID can be found in the URL… for example:
https://steamcommunity.com/sharedfiles/filedetails/?id=507600288 <-- this number is the collection ID.
When you load your server, it’ll automatically download all the addons. Please note there is a file size limit that the server can download itself. If an addon is too big ( it’ll say in the console ), subscribe to it yourself, then go to your local garrysmod/garrysmod/addons folder, take the addon, and change the addon file (.gma) to ds_addonid.gma. Then upload that yourself to your servers addon directory.
Finally, on your server, goto lua/autorun/server and create a file. It can be called anything, I usually call mine resources.lua. ( must end in .lua )
Then for each addon in the collection, add a line:
resource.AddWorkshop(“175983422”) <-- the addon ID which can be found in the URL of the addons page on the workshop.
This resources.lua will make sure clients download the workshop addon as they connect.
Hope this helps!