I think I have actually found the answer through Google ending at the METROBUS website. At this
sub-page, it says:
"HOLIDAY INN GATWICK AIRPORT HOTEL (Povey Cross Road, Horley)
From Gatwick Airport Station/South Terminal catch route 100 towards Redhill or route 20 towards Horley (follow signs to 'Local Buses' from Gatwick Airport railway station) and alight at Povey Cross Road East. Adult single fare £1.60."