I found this:
Is it possible to get a 6.0 GPA?
Not All GPAs are Created Equal
There is wide variability in the scale used to calculate GPA. GPAs can be based on a 4.0, 5.0 or 6.0 scale.
And this:
There are two types of GPA scales (weighted and unweighted), and they work differently. An unweighted GPA scale is based on a 4.0 scale. Regardless of the type of course, regular or advanced, an A is worth 4 points. Bs are worth 3 points. Cs are worth 2 points. Ds are worth 1 point. Fs aren’t worth any points. When using an unweighted GPA scale, the highest GPA one can receive is a 4.0. In a weighted GPA scale, Advanced Placement (AP) or Honors Courses are worth more than regular courses. In a weighted GPA scale, an A is worth 5 points. Bs are worth 4 points. Cs are worth 3 points. Ds are worth 2 points. Fs are not worth any points. When using a weighted GPA scale, GPAs can be higher than a 4.0, possibly all the way up to a 5.0