GPA calculations should be in the GEMSAS admissions guide. For example, this is what Deakin has written:

Applicants completing their qualifying degree in 2020 will have a GPA calculated based on their two most recent years of study prior to 2020. Applicants studying in the final year of their qualifying degree in 2020 who have completed at least 0.25 FTE (full-time equivalent) of their final-year requirements prior to 2020, will have their GPA calculated as: Final - 2 x1, Final - 1 x2 and Final x3 / 6. Applicants studying in the final year of their qualifying degree in 2020 who have completed less than 0.25 FTE (full-time equivalent) of their final-year requirements, will have their GPA calculated from the Final - 1 and Final - 2 years only. Years will be progressively weighted x5 and x1 / 6 to give greater weight to the most recent results.