Good luck, that's not a common item at all. My recollection (from an old an feeble mind) was that the Gwinn was an AR-18 knock off. Bushmaster bought the rifle design and turned it into the Bushmaster "arm pistol". So if you can find a manual for a Bushmaster Arm Pistol, you'll be close: Bushmaster Armpistol
I saw one of the Gwinn rifles at a gun show once and thought it was a home made rifle. Never got to take it apart or shoot it, just held it and looked at it.