There are several bands named GTO:

1) One of the many aliases used by Michael Wells and Lee Newman.

"Lee Newman and Michael Wells formed Greater Than One in London, circa 1985. The group was created to explore art through performance, recordings, and installations. They released albums on Graeme Revell's Side Effects label, WaxTrax! and ROIR in the USA, Torso in Europe, and their own imprint, K=K.

In 1990, they started releasing singles as GTO (because it was easier to spell) and Tricky Disco (to explore more techno and house music). Some of the hits were Pure and Elevation. As the rave scene of 1991 exploded, Newman and Wells started releasing singles as John + Julie, Church of Extacy and Signs of Chaos. They would later release albums and singles as TD5, the salami brothers, and Technohead.

In 1995, Technohead scored a massive worldwide hit single with I Wanna Be a Hippy. In August however, Lee Newman passed away after a brief battle with cancer.

Michael Wells continues to DJ occasionally and record music under various names including S.O.L.O. and The Man as well as many of the above." (excerpt courtesy of discogs)

2) Assumed name for several OSTs of a japanese anime called 'Great Teacher Onizuka' aka GTO

3)