Are you using physics behavior?
1st off, you need to create a family (you can call it physics objects or whatever). Add any objects that will be affected by gravity. Add physics behavior to the family and set it up for however you want the defaults to be. In my projects I have a group that is called initialization I have it at the top of my event list and at the end of the group I have a turn group off action. Everything in this group will run only once and then never again unless reactivated. In this I select a physics object and set the global physics setting gravity to 0.
Then, somewhere in your event list you need an action that applies the force of gravity to your objects at whatever angle you need gravity to be at that moment. It looks like this:
every tick -> family.physicsObjects apply force (objects mass * desired speed*dt) at angle (This would be the angle of gravity (down is 90)).
This is the way I do it and it works nicely.