If it has brakes, you can tow it. As it can't have a load put on it, I would assume it counts as an unladen trailer which you can tow with the GMC as a test exempt vehicle. They aren't very heavy though, I've seen heavier items pulled by GMC's without brakes before! The 105mm howitzer originally had brakes but the later models had none and they were designed to be pulled by the CCKW-352 and I think the Bofors is a similar weight, perhaps a little more.
Given to you? Blimey, I wish I had luck like that! :thumbsup: Like to see some pics when you can...