Thanks for playing! We're glad that you enjoyed it.
The costume changes were purely cosmetic. We had originally toyed with them being an achievement based unlock system, but we had to pare that down for the jam.
The gravity boots were used mostly in the fuel bay room so you didn't have to recoil into the green and die. They were also for QoL moving around the base (fast travel by shooting yourself down a corridor and then stopping outside the room you want to do next).