Wow, crazy!
I don't know how to interpret that. They should have just made this work like the Shadow Conjuration spell line, to make it easier to interpret. Unseen Servant isn't even an illusion! I guess it might be possible to disbelieve it, but if you did so, you'd see... an invisible unseen servant? Hell if I know...
If I think of something else that Gabriel should do before leaving with Arkesh, I'll post it, but I can't think of anything right now. Getting a tougher, more durable body to ride around in is probably a good idea, though...
Hmm... how about something fairly simple like the
troll hunter in the SRD? It would have to roll a 19 or 20 to resist Gabriel's Magic Jar. Once he had possessed it, he could chain it up somewhere, move to another body, and cast Necrotic Cyst and Necrotic Tumor on it till it failed both saves. Then it would be permanently enslaved, so he wouldn't have to worry about it attacking his allies when he leaves its body.
He wouldn't want to take his factor's body and get it killed. So he'd probably have some kind of "combat body" somewhere he uses when needed.
Sound OK?