To start with, don't let box2d handle gravity:
On start of Layout -> Set physics object World Gravity to 0
Then add everything you want to be affected by gravity into a family and call it "anything".
Everytick -> "anything" Set physics velocity to whatever you like per frame.
You can get fancy and have gravity change strength, angle or both over time. Really, if you manipulate velocity rather than adding forces or impulses you can get very predictable results without compromising the integrity of the physics world. Using this method you can even have a max fall velocity.