Egg Gauntlet Zone is a Zone intended for use in the 2013 re-release of Sonic the Hedgehog 2. It was meant to serve as a boss attack mode, but was scrapped in favor of the original boss arenas that became Boss Attack Zone.
Egg Gauntlet Zone was the first attempt at a boss attack mode for the 2013 re-release of Sonic the Hedgehog 2. The Zone featured original mechanical scenery with a drab gray color palette. Sonic Team, however, preferred the original boss arenas, and so the Zone was replaced by a Zone in which the original arenas were placed back-to-back with no significant changes.
Basically during development, we tried something different for the Boss Attack mode. Instead of the original bosses back to back, they were all in the one zone with connecting platforming sections - the idea being that it was a testing facility, with mechanical palm trees, flashing lights etc etc. It was fairly close to completion (it just needed a few graphical effects, replacing placeholder graphics, and general polish), however Sonic Team didn't agree with changing the location, colours or graphic style for the Bosses.
The mechanical scenery of Egg Gauntlet Zone seems to use graphics similar to the ones used in one of the levels of the scrapped Sonic game Sonic Crackers.
This Zone is the most complete of the scrapped Sonic the Hedgehog 2 Zones, as it contains a full level layout complete with objects, suggesting that it was removed late in development.