In the prototype build we have of the Game Gear conversion of Sonic the Hedgehog, only Green Hill zone 1 is playable. However with modern tools, now we can bypass this restriction and access the rest of the game.
A writeup of all of the different zones accessible and their behaviours in the unmodified ROM can be found here. This page focuses on Green Hill zone 2.
In the unmodified ROM, trying to access the second zone of Green Hill through a cheat tool (even if the title card reset is disabled, finishing a zone doesn't move you to the next one) will hang on a black screen. This can, however, be fixed by changing ROM address $155FE from $C4 to $B6 (thank you, Alex Field!). It will spawn Sonic safely in Green Hill zone 2.
The first difference here is that flower that seems to be floating in mid-air. Some of the background elements are broken here in this build.
Also, if Sonic moves to the right and falls in, he just falls down like he fell off a cliff, instead of using his hurt pose and bouncing up before falling in like in the final game.
The spikes also are glitched, using otherwise-unused graphics. It seems the spike objects' graphics were overwritten with these unused tiles for zone 2 only, as zone 1 does not contain them.
It's still possible to glitch your way in here.
A weird difference - this ring monitor obviously is not there in the Master System build. It can be broken like normal, and will lead into the path toward the Chaos Emerald for this zone, which is also fully operational.
This platform seems misplaced. It doesn't go far enough left, and goes too far to the right.
Cloud flower
When using the ramp and falling down here, a ring box seems to move up quickly. Coming back up there reveals it's parked on the floor.
Entering the Special Stage from here (which is only possible if you have the reset procedure patched out, of course), you'll spawn in the air and die, getting reset back to the map screen.