Grammar Question: "GPAs"

In an update letter to admissions, I'm wanting to say, in grammatically correct form: My science and cumulative GPAs increased to 3.9 out of 4.0. What is the most appropriate way to spell "GPAs?" Should I write it out, "grade point average;" should I use an apostrophe, "GPA's?" Thanks.

Source: Student Doctor Network - January 12, 2018 Category: Universities & Medical Training Authors: Gauss44 Source Type: forums