I´m giving you some feedback I think may be helpful:
1. You can integrate both buttons for the gravity change into one only key at the keyboard to activate or deactivate reversal gravity. Easier to understand. And usable also when the player is on the ground, not only when you are in the air.
2. It takes a lot to get the gravity to make effect. Get sure to make the platform puzzles with this timing in mind.
4. I like the idea of power-ups. Maybe make some temporary items or power-ups to solve puzzles at some points.
By the way, I´ve replayed the game. This time when I get the power-ups, the shield worked, but the first time doesn´t work. I´m at the final boss, but I don´t know how to throw the blocks to beat him.