1. Under your drum loop track, pull in a MIDI loop of drum beats -- does not matter what it sounds like.
2. Begin the track at count 1 and extend past to where your loop ended abruptly.
3. Do NOT delete all MIDI notes (yet); in GarageBand, a track with no notes often causes a problem.
4. At the place where the original drum loop track ends abruptly, add a MIDI sound on the MIDI track. I like to use crashes or other 'ending' sounds that will blend with the sound of the original. This MIDI sound will extend the drum sound to be more natural, tapering off as live drums do.
5. Now delete the unwanted imported MIDI notes.
6. Remember to adjust the volume of the MIDI note(s)
to match the sound of the original loop.