To set the gravity of an object, you can use the "set gravity" action from the "move" tab, or in a create event you can write :
gravity = ?*;
?* = number you wish to set it to.
Gravity is quite sensitive so its best to use lower numbers.
you need to set the gravity direction too...
You will use °
270° <+> 90°
so you can put this
(At step event) if place_free(0,self+1) then gravity=?* else gravity=0(if you want)