dumb question but how do you change your WAM to a GPA?
Actually fairly complicated because they're calculated differently.
WAM is done by taking the average of your scores, however are weighted by the credit points. Formula/examples available
here.
GPA, however, can be done in two ways depending on the uni (WAM also differs, but Melbourne has a WAM calculation, not a GPA calculation, so I'll only explain GPA differences.

), although they're not terribly different. The main two systems are a 4 point or 7 point system.
In each system, a letter grade is assigned a specific amount of points - for the 4 point system:
High distinction 4.0
Distinction 3.0
Credit 2.0
Pass 1.0
Near pass 0.7
Fail 0.3
Withdrawn fail 0.0
For a 7 point system:
HD High Distinction 7
D Distinction 6
C Credit 5
P Pass 4
F Fail 0
These points again can differ between unis - the ones I grabbed are from Monash (4 point system) and Newcastle (7 point system)
This is once again an average, but not of your actual percentage scores - instead, of your letter scores. So for example, in my first semester, I got scores of 87, 78, 69, and 60. This gave me an >70 WAM, but a GPA of 2.5 - so, the WAM suggested a disctinction student, but the GPA said I was a credit student. To actually find out your GPA, you'd have to calculate it yourself by using the method above.