Hey there

I've seen many people use the GECK to change weapons, or NPC's and I was just wondering how to change what a faction is wearing.
I've loaded up the faction menu, but it wont let me change whats in their inventory.

Basically I want to change the Slavers from all wearing Merc outfits to all wearing Chinese Jumpsuits, to give them a more military look

This is not done from the faction menu. You edit the NPC record itself, in the NPC category of the GECK - although frequently, NPCs use a "base" npc sort of like a template, you would edit that one. Look for stuff with "slaver" in the name, perhaps, and then when you find the right NPC records, their "inventory" is what you want to see.

This doesn't have a screenshot of what the place looks like but once you find the right stuff to edit, it describes it: http://geck.gamesas.com/index.php/Category:NPC