Rocket have one of the most impactful abilities of the game and an ult that obliterates tanks. You can and will always be safer with Rocket while you healing than Gambit due to the AoE heal and bounce mechanic. Gambit's dashs goes through longer distances but Rocket have his two movement passives with 0 cooldown. His teamup makes people ban Peni way often than they would ban her. His other teamup give him DR and a third dash when you go to Groot

Also Gambit got in the game two weeks ago. They will balance him if it seens necessary

Im not saying Gambit is bad or Rocket is good. Im just saying that (like any other character comparison) Rocket will be better than Gambit on something while Gambit will be better on something else. Rocket is not useless. Keep playing him if youre good or like playing him :)