Grade point average (GPA) cannot easily be converted into a measure of academic achievement (AA) because they are two very different things. GPA is simply an average of all of the grades you have received over the period of interest. It is a overly simplified measure that correlates loosely with AA. But the correlation is not very good. At the top It can not distinguish between very good students and exceptional students very well - but a good teacher can. In the middle it usually doesn't account for strength of schedule/class difficulty very well and at the bottom it also usually is not an accurate measure of relative abilities of students. But just because it can't be done doesn't mean that somebody hasn't tried and possibly even had some limited success.
GPA of a small group of students who all take the same classes may have some meaning as a means of measuring who is better or worse at the skills required for that particular class.