From today's NSW Education Department website reflecting the commentary of Kerry Chant of last week and repeated again today:
"Interschool activities must remain within the local community or zone. This applies to all interschool sport and physical activities, curriculum activities, arts activities and extra-curricular activities organised or sponsored by the school. Schools must not travel outside of their local community area. Metropolitan schools cannot travel outside their local area or to regional areas. Regional areas cannot travel into metropolitan areas or other regional areas".
Unless the GPS has managed to obtain a specific exemption for GPS Sport, its hard to see how most GPS schools (with the possible exceptions of Joeys/Riverview and Grammar/Scots) would be regarded as being in the same local community area.
Interestingly while schools are not permitted to travel out of area, community sport doesn't seem to have the same restrictions, eg SJRU typically has teams travelling across Sydney to play each other.