Change Gravity

I’m trying to make a game where at the start of say the 3rd level, the gravity is reversed (IE -1.0) from the other levels (initially gravity = 1.0). The idea is to solve a puzzle (IE press a button/trigger event) to return gravity back to normal (1.0). I’ve so far been unable to figure out how to affect gravity at all.

Any help would be appreciated - I have tried the “Fake Gravity” thing, but it doesn’t do anything at all.