PS: Your cursed
https://imgur.com/YOXYAo4

For anyone interested, I have figured out how to include Workshop NPCs and how to signal to Hammer on how to pull reference them.

You'll want to unpack your NPC addon of choice with gmad.exe found in the Garrysmod Bin Folder.

When the respective addon's folder is generated in your garrysmod/addons folder, open the lua folder and find the script for your NPC of choice.

In there you'll be looking for a class name similar to the Default NPC classes seen in Hammer such as "npc_soldiercombines" or "npc_citizen".

In my case, it was "npc_alfie_odst_friendly".

Copy that classname and paste in the NPC Classname section of your NPC Spawnplatform properties.

Save and Compile your map and load in Garry's Mod and VOILA!

Hope this helps anyone who wants to build these into their map and not be limited to HL2 NPCs.

I'll post more as more becomes revealed through trial and error, such as a custom weapon class and some ability to modify bodygroups.

Stay posted.

Ok same deal with weapons guys and gals:

Unpack the NPC Swep addon of your choice and head to the lua for the weapon classes.

There you'll find a list of all the various weapons that addon references and, same drill, copy and paste into the Weapon Class section of your Spawn Platform properties menu within Hammer.

Save, Compile and Load the map and once again, works like a charm.

In case it isn't already clear, make sure to actually have the addon installed in your addons folder and make sure your referencing NPC SWEPS not player weapons or props!

Good luck!

If I can come up with a work around for Bodygroups you'll find it below in time.

I'm glad this is working for you, this feature has always been somewhat experimental. I don't currently have any way to set body groups (or spawn flags). Happy to help with other problems