Your gateway band is the band you first listened to which completely opened up your world of music.
The band which was the beginning of you developing a decent taste in music.
Indie kids often use it to describe the first indie band they got into. But this term can apply to particular genre of music.
Guy-who-only-listens-to-commercial-rock: *listening to Nickelback*
Me: *hands him a bright eyes cd* Put down that rubbish, listen to this, and enjoy.
Guy-who-only-listens-to-commercial-rock: *listens to Bright Eyes* OMG, this is amazing! *stomps on Nickelback*
He then turns into either a pretentious indie kid, or just develops a decent taste in music. Yay
Bright Eyes was his gateway band.