Besides making it insanely easy to reach Mars, gravity kill does absolutely nothing else special for you; no special endings or anything. And by the time you get enough coins for it, there is no way you could not have reached mars by then. Useless upgrade.
I say that because I have a bunch of guns and bumpers at one end, and one turtle inevitably gets stuck bouncing at the end for an eternity before falling off to the right...
It's really easy to do the speed tests.... right-click is blocked, but there are other methods to cheat.... Like win key+M, minimize all windows; move cursor to end position, alt-tab to game, win, repeat.
 "Ball Revamped 4"){kind=icon}
 "Bomboozle 2"){kind=icon}
 "Anbot"){kind=icon}
 "Flaming Zombooka 2"){kind=icon}
 "Me And The Key 2"){kind=icon}
 "Gravitee"){kind=icon}
 "Gravitee Wars"){kind=icon}
 "Hedgehog Launch 2"){kind=icon}
 "IntoSpace!"){kind=icon}
 "Lucky Tower"){kind=icon}
 "Ultimate Assassin 2"){kind=icon}
 "Road of the Dead"){kind=icon}
 "Epic Combo!"){kind=icon}
 "How is your mouse"){kind=icon}
 "Underworld Army, Episode-1"){kind=icon}
 "The Missile Game 3D HS"){kind=icon}
 "Reachin' Pichin"){kind=icon}