From Sonic Retro
Space Gadget one of five possible fifth levels of Shadow the Hedgehog; in the game's branching path structure, it follows taking a somewhat heroic, G.U.N. / Sonic-aligned path through the previous levels. In a recreation of Sonic Adventure 2's Final Rush (Sonic even says "This is just like old times"), the black and blue hedgehogs sprint through the Space Colony ARK, cutting through GUN and Black Arms crossfire in a race to recover the cyan Chaos Emerald.
The level is filled with gravity-switching devices, both inside and outside the ARK proper. As such, Shadow will often find himself walking along ceilings or corridor walls. Some of the interior rooms form mini puzzles, as Shadow has to find the appropriate gravity switch to allow him to progress.
This is the only level in Shadow the Hedgehog where one mission can't be completed until another is failed. Getting to the Emerald in less than five minutes completes the Hero path; if you want to go the Neutral route Shadow has to stand around until the Hero route is failed by Sonic snatching the gem first. Shadow also follows an almost entirely different route through the stage if taking the Dark path.
Chaos Controlling into Iron Jungle, Shadow is greeted not by a batallion of Shadow Androids (as in the full Neutral route), but rather by Sonic the Hedgehog, riding the nose cone of Tails' Sea-Fox-esque spaceship. The Blue Blur reports that the Black Arms are retreating back into space, and offers Shadow a ride to the ARK.
GUN and the aliens are fighting a fierce battle for possession of the orbital ruins. Shadow can either play along with Sonic and help him keep the Chaos Emerald out of the wrong hands, or side with Doom's Eye and destroy the six green shield generators to prevent the humans from re-occupying the ARK.