This word is used to describe an un-cool person (Usually a guy) who becomes even more un-cool when seen standing next to, in the car with, or eating dinner with an incredibly hot chick. A gankus usually knows the girl because of family relationship or because she is a co-worker. He does not deserve her, nor should he be in her presence. He has no chance with any chick, except for a gankus girl.
Dude, look at that gankus over there. She needs to drop that shit and come over here with us, son.