Update: The above info from other players appears to be incorrect. You can have items changed to look like Gargoyle items, but your gargoyle won't be able to wear it.
Dante changed my Mirror shield to a Gargish Order Shield, and it wouldn't let me equip it. I got the message 'Gargoyles Can't Wear This'. With that said, it would be a cool service to have in the future!
At first glance it would appear that you would be very gimped. I was bored so I made a gargoyle yesterday. Unless there are slots that I'm not seeing available, the ability to layer clothes is not on par with a human or elf. Even with spending evo dollars to convert items to gargish equivalent, it looks like Gargoyles have 3-4 less slots, which presents a serious parity issue currently. I could be wrong, and be missing something. I'll do some more research tonight, and let you know if I find anything out.