The GAMSAT Score requirement is different for each university, some have a cut-off, others have a ranking system. Make sure to find out how the GAMSAT is assessed by your university choices. I would recommend having a look at how all the universities use the GAMSAT to invite candidates for interview, use this information to create a plan of attack. For instance, if you achieve your expected score in all sections you can apply to your first choices, however, if you score below your target score you can consider other universities as a backup that has a lower GAMSAT requirement and will more likely invite you for an interview.
You can download an up to date copy of the GAMSAT Score Requirement guide here. I recommend using the information provided to set a target score, also check the additional entry requirements for each course.
For more information on the GAMSAT check the frequently asked Questions (FAQ).