Yes it is possible.
I use a dedicated gameserver from NFO because IMHO its cheap enough to avoid the headache of setting up a server yourself.

WHAT YOU NEED:
---Access to a GMod gameserver's filesystem.
---GMad (too lazy to link but its a well known tool for extracting workshop .gma packages into raw folders to be added to a server)

First subscribe to all the stuff you want on your server (there are 9 maps that work out of the box)

Next make a new Collection in the workshop and add all the maps you want and anything else)
You’ll have to go through the whole steam api deal, that comes into play later.

Now take each workshop item in your collection, and add it to your "FastDL"
(make a lua file called workshop.lua and put it in garrysmod/lua/autorun/server Each line should have

resource.AddWorkshop("insert workshop item id here")

next you will find the .gma files for each map you want to add, and use GMad to extract them. Put the contents into garrysmod/maps

Slender automatically adds the maps it recognizes so all thats left is to get the gamemode onto the server

extract the stop_it_slender_gameode.gma (its called something different, but its the main Stop It Slender! thing this discussion thread is based on obviously) and convert it with GMad

Put the files where they belong on the server, you should be getting the hang of it by now.

now change the gamemode in your autorun.cfg or whatever and BOOM! slender server.

Its not rocket science, but there isn’t much documentation on this stuff so anyone who needs help feel free to message or add me for support. Ill try, no promises

WEhere do i put the Files, after extracting them from the .gma file?

For anyone else looking to do this... Honestly, if you just create a collection on the workshop (it can be private if you want) and then start your game with +host_workshop_collection <collection id> it'll do all the file stuff for you, no extracting or moving.

My dedicated server starts like this...
-game garrysmod +gamemode stopitslender +maxplayers 16 +map slender_forest +host_workshop_collection 1186243767

That's it. No extracting files necessary. You can even use that collection if you want, but you'll be running whatever maps and mods I run on mine. :)