From Sonic Retro
Sonic 6 is an unlicensed "pirate" game for the Gameboy supposedly released in 1998. It is actually a minor graphical hack for the 1993 Speedy Gonzales Game Boy cartridge by Sunsoft, replacing all sprites and references to Speedy Gonzales with Sonic. The game was likely chosen to hack due to Speedy Gonzales' similar gameplay mechanics to that of the Sonic series.
Unlike the original game, which starts in "Ice Zone," Sonic 6 starts with "Forest Zone". The game is credited as being made by "Sonic World", and is believed to be the same people who pirated Sonic the Hedgehog on the Super Nintendo. It utilizes a password system. Also, Instead of the Cartridge saying "GAME BOY" at the top, it says "GAME".
Below is a list of zones and passwords for Sonic 6 which are different from speedy gonzales.
Images shown on the right are from the original Speedy Gonzales, the left from Sonic 6.