I spent six wonderful years living in Manchester between 2004 and 2010. The Blues fans I met during that time were indeed "real supporters" as you describe, many of whom had been at the game and were sympathetic to a Gills' viewpoint.
At the same time virtually every United fan responded with "Who?" when Gillingham came up in conversation.
Loved the city and the people, it's a great place. So many local away games too, I took in Oldham, Bury, Rochdale, Accy, Stockport, Blackpool, Bradford, Macclesfield, Port Vale and Burscough (*shudder) during my time up there.