Sonic the Hedgehog 2 Beta - Game Gear

Here is the title screen, lacking Tails.

The most profound difference between this and the final version is Green Hill Zone. It has the appearance of the level seen in the intro and outro sequences, with grey squares instead of brown "Tetris" blocks (which the final version of GHZ had).

Full Level Map 1 ~ Map 2

Sky High Zone.

Underground Zone (act 3). Different layout, spikes are missing in the beta.