If we wanna go by the series' definition, only Kira Yamato and his gang call it gundams. No one else does because the term is coined only by him. I doubt even the ppl who came up with the original O.S calls it Gundam. Bear in mind the original Gundam O.S is damn lousy until it may not be possible to pilot them, until the coordinators reprogrammed them into something more user friendly. So using O.S as gauge of Gundam or not is kinda moot as well....since when Kira 1s saw the Destiny, he straight away say "GUNDAM?!" when he never see the O.S before. Its just the overall look and it stands out as a hero unit as you said.

To me U.C Gundams are the "concept car" of Gundam world...prototypes before they become into mass production models called GMs.