"Set angle of gravity" does exist though
Unless you mean in general, in which case what you can do is create a family of all the objects you want gravity to be applied to, and then set the angle of gravity of that family.
Edit: ah I miss read you the first time. However what you could do is make a family then apply the platform behavior to everything and turn off player control.
Edit2: or not because you cant control platform behavior of families haha silly me
Edit3: oh hold on, so apparently you can apply behaviors to the families themselves.
So what you can try is put everything you want to change gravity of into a family. Apply the platform behavior with default controls turned off to that family. Then add an event that sets the angle of gravity of that family. Hopefully that works.