Google lists Koehlers Wild Game and Custom Processing in Gillette.
I haven't used them, I did use one in Lusk, WY once but that's still a long way from Gillette, a couple hour drive or more.
Anytime you use a wild game processor in a hunting area be prepared to pay a double price compared to what you may pay at home. I still use them even with the high price. It's just easy to get everything taken care of before you get on the road for two days.