Lucie Miller once refered to Gordon as "Flash Gordon". (AUDIO: No More Lies)
Whilst in their spacesuits, Evelyn Smythe called the Doctor "Flash Gordon" and Thomas Brewster "Buck Rogers". (AUDIO: The Feast of Axos)
Behind the scenes
Crusty Cobbe was named after Buster Crabbe who played Flash Gordon and Buck Rogers in serials in the 1930s and the 1940s.
The Lion-Men of Mongo were mentioned in Invasion of the Cat-People by the Seventh Doctor, who are also characters from the Flash Gordon franchise in the real world, though this is not made explicit in the DWU.