Zone 66 is a top down shooting video game released in 1993 for DOS as shareware. The game was created by a North American Demo Scene group called Renaissance, and was published by Epic MegaGames (now Epic). Because demoscene programmers frequently worked to maximize hardware performance, the group developed a 386 protected-mode (PMODE) extender to enable the game to play on a 386 processor clocked at 16 MHz with full screen updates.
Two different soundtracks were available for the game. The first, included with the game, was the Sound Blaster soundtrack, consisting of a mix of FM synth and tracked samples. This particular method of music creation and playback was unique to the Renaissance demogroup.
A second, separately downloadable soundtrack was available for owners of the Gravis Ultrasound sound card featuring high quality music. Some songs in this soundtrack, such as the title theme, are completely different, while others are different arrangements of the same compositions.
The reason for the separate "GUS" music download was due to the size of the Gravis music: as it was nearly as large as the original game, users who only had a Sound Blaster didn't have to waste time downloading music they would never hear. Disc versions of the game included both, as players did not need to download the game.
Source: Wikipedia, "Zone 66", available under the CC-BY-SA License.