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1.
When you do something amazing that would normally take more than 4-5 people.
In Modern Warfare 2:

Guy 1: "Hey you just killed the entire enemy team in Ground War!"

Guy 2: "I know, I'm a one man gangbang!"
by DoctorGoodWood May 27, 2010
 
2.
When one man has anal, vaginal, and oral sex with one woman during one session, in any order. Also known as the trisexta.
Will: Want to go the bar tonight?
Bill: Not before 11, I am going to one man gang bang my girl in a bit.
by DrSussy October 07, 2009