Are you using any behaviors on the enemies or just moving them from events?
If so, you can have an event "For Each Object" from the System events and select the object you want to fall, then add a condition saying "Is overlapping" and select your ground object or the "Solid" attribute.
Next, right click this condition and choose "Invert". You can then add an event to increase the Y position of your object by a few pixels (eg: 5, or 10 for a faster fall speed)