This is a Sonic 1 hack made by JoenickROS, with sprites originally made by PacManfan64 and Gecko1993, and additional coding by MarkeyJester and Natsumi.

This build is the most recent version released, originally seen at SHC2020, and must be patched to the Rev0 version of Sonic the Hedgehog to work properly (patching to Rev1 crashes the game upon booting up).

This hack attempts to implement gameplay from Gex (1995), meaning the controls are slightly different than normal (minus wall climbing, which hasn’t been implemented yet):

C - Jump / Swim Underwater (hold Down in-air to Tail Bounce, pressing Up immediately after a Tail Bounce increases height)
B - Tongue Lash / Use Power-Up (hold B to Run, Tongue Lash can also be used when looking up, in-air or crouching)
A - Tail Whip (can also be used in-air and when crouching)

Just like in the original Gex games, you must find a TV remote to progress which is hidden somewhere in each act (including boss acts), so exploration and visiting different areas and routes is key to completing the game. Neither signposts nor any bosses will spawn without it, except for the Final Zone boss. There’s also yellow remotes that give access to Special Stages, replacing the 50 rings requirement, but you’ll still need the normal stage remote.

In addition to the altered gameplay, there’s also Power-Ups. Eating one of these grants Gex with the following until he gets hit:

Paw Print - Adds one hit point to Gex’s hit counter (located at the top right corner of the screen, maximum is 6).
Ladybug - Replenishes Gex’s hit point counter to the maximum amount.
Firefly (red) - Gives Gex the ability to shoot a fireball from his mouth while tongue lashing. Using this underwater makes Gex blow air bubbles instead, and inhalable ones can only be spawned when tongue lashing to the left.
Lightning Bug (yellow) - Gives Gex the ability to shoot a wave of 3 lighting bolts from his mouth while tongue lashing.
Frost Fly (blue) - Gives Gex the ability to shoot an ice ball from his mouth while tongue lashing. If it hits an enemy, they will be frozen for a short period, allowing Gex to use it as a platform until it’s attacked or explodes on its own.
Centipede - Gives Gex extra speed for a short period of time.
Caterpillar - Gives Gex Invincibility for a short period of time.
Butterfly - Gives Gex one extra life.

The manual provided, made by Gecko1993, gives you hints to where the act remotes are, though only if needed.
Remote locations start at page 14 in the .pdf.

Credits:

JoenickROS:	Main coder
PacManfan64:	Gex sprites, Misc. sprites, Credits sprite mapping
Gecko1993:	Original drafts for the title screen animation, Remote sprites, Manual, In-Game Credits text
Natsumi:	AMPS Sound Driver, Bug fixes after sound driver installation
MarkeyJester:	Additional Run frame code, Dual PCM