I was talking to a friend recently who was making a journey back from Brighton to Guildford and encountered North Downs Line services terminating at Redhill as a result of various disruption on the Brighton mainline.
He has an
ENCTS▸ pass so caught a service bus to Redhill from Gatwick which took an age. Turned out to be the 100 Metrobus route from Maidenbower to Redhill.
I took at look at Traveline timetable search for Gatwick Airport under "Location" to see what other buses went from the airport (I don't think you can bookmark URLs/searches that easily)
http://www.travelinesoutheast.org.uk/se/XSLT_SELTT_REQUEST?language=en...to my surprise the South/North inter-terminal shuttle has a timetable listed!