There are multiple artists using this name:
1) A British progressive jazz-rock band of the Canterbury style in the 70s.
2) The incorrect English spelling of the Japanese rock band ギルガメッシュ (girugamesh).
3) A now defunct Dutch hardcore band.
4) A now defunct Australian ska/hardcore band.
5) A progressive rock/fusion band out of Petaluma, California.
6) An american progessive band. Their demo is still under production, however, some songs may be found at www.purevolume.com/gilgameshusa.
7) A now defunct Hungarian lo-fi no-wave art-punk band.

Read more about Gilgamesh on Last.fm.