U will need a QF asic. But that is done in house and you don't have to pay.
You do get penalty rates however most overtime goes to the old Qantas workers first before flowing down to QGS
Good thing is that once you are working within qantas there is more opportunity job wise transferring out of QGS into Qantas which is encouraged.
Being successful at an interview for the various positions well that's another topic within itself.