From the back of the Game Box:
Play the Original that Started it all!
The original Sonic the Hedgehog that broke the speed barrier wide open on the Sega Genesis is even faster now with Sonic's super speedy Spin Dash! As you rocket through the seven original stages, including the celebrated Green Hill Zone, past bubbling lava and on, you'll unlock songs from the original and for the first time, save your progress and high scores.
Celebrate Sonic's 15th birthday with Spin Dash in Anniversary Mode!
Summary from Sonic Retro page:
Sonic the Hedgehog Genesis is a Game Boy Advance remake of the original Sonic the Hedgehog for the Sega Genesis, made in celebration of the 15th anniversary of that game, as well as Sonic the Hedgehog himself.
The game is criticized for its slowdown (for example, obtaining speed shoes causes the game to slow down at times, rather than speed up), poor music renditions and sound quality, unfaithfully ported movement physics, shortened and different ending sequences, and numerous bugs and glitches. This has led it to be considered one of the worst Sonic games released.