Gravity is one of the "built in" properties all instances have and, when set, will apply a constant force in the gravity direction of the instance, influencing its speed. Note that gravity is a cumulative force and will accelerate the object if you choose not to cap the final speed.
Real (single precision floating point value)
if !place_meeting(x, y + 1, obj_Ground)
gravity = 0.01;
gravity = 0;
The above code will only apply gravity if the instance does not find any instances of "obj_Ground" beneath it.