[Halbarad]

If Sauron did not trust men, why was the lieutenant of Barad Dur a man who was deep in his plots? In the whole history of the Ring never once did a being already in the service of Sauron handle it. We cannot know (don't get all post-modern on me now) whether a servant of Sauron would have claimed the ring as his own. The greatest power the Ulairi had was fear; complete and utter fear, and so fear of Sauron would have had a slave or a servant return the ring to its master.
Back to my point. If Gothmog was indeed a man I doubt that Sauron would have had much fear of him claiming the Ring. He feared Aragorn or some other powerful Lord takign the Ring and using it against him, and let's not forget that Aragorn was a powerful Man, and Gothmog, if he was a man, would not have had the same innate power for the Ring to give power to. So, Gothmog could have been a man.