Gympie is a common waypoint for aircraft traveling north from Brisbane, Caboolture, Redcliffe and Caloundra being west of Maroochydore airspace and has active local traffic.
The accident apparently occurred 16km (9nm) from Gympie at altitude. If the Sabre was transiting the Pilot would possibly just have switched to ctaf from area. A glider at same altitude does not offer much of a profile and at possibly 100+ knots closing speed……… my 2 cents.