I would say it depends on the show. If you're at an Arabian breed show in a 1/2 Arab halter class, then show him Arab and groom him accordingly. I would not remove feathers though, that's part of his heritage. In an open show, I'd show him as whatever breed looks dominant in him. If he looks Arab-y I'd do that, Gypsy then I'd show him however they'd show at a breed show. Do Gypsy's have a 1/2 registry or is their book still open to Arabs and you have a purebred Gypsy colt according to their rules?