@Ricochet
Make sure you're using citizentype4 (unique) and make sure the model paths are correct. If I remember correctly, Gordon's looks something like models/arbeitdudes/gordon_citizen.mdl

@xdxdxd Hello and sorry, I was testing the spawns, they worked perfectly changing the skin with your help, but I have another problem, when I spawn Gordon Freeman in a citizen a giant error appears, I tried several ways but it remains the same, it only happens with gordon_citizen.mdl the rest works, it also happens with the model with a helmet and I can't make the assassin invisible, I don't know what I'm doing wrong but I would really appreciate it if you could explain it to me, if you want and if you can of course, there's no rush

or well, if you want to change the skin of an existing npc, i think it would be "ent_fire !picker skin 4" while looking at the npc

@Ricochet
Both work. In Hammer, you can change the skin slot either in the model viewer or entity properties. In the console, when using ent_create, you can add the keyvalue "skin" and then the number of the skin. example: "skin 2" would spawn in the Mark4.
if i remember correctly, this is the order of the skins. 0: Mark5, 1: Mark5 Bloodied, 2: Mark4, 3: Mark4 Bloodied, 4: Arbeit, 5: Arbeit Bloodied, 6: Arbeit Glowy

One question, to change the HEV suit skin, do I need to use the console? Or is the Hammer Editor required? I'd really appreciate it. I'll give you credit.

Small but somewhat relevant update:
Helmet's helmet skin now works via skins, no more bodygroups
Gordon now has a bodygroup to remove the glasses

into the borealis

hunt down the freeman 2

gordon boss fight when /j