Does it give letter grades, or does it use narrative evaluations or some such without letter grades?
If it uses letter grades, calculating GPA is not a difficult task. For each course with a letter grade, multiply the grade points for the grade by the number of credits for the course. Add all of these products together and divide the sum by the number of credits of letter graded courses.
Grade points would be the usual A = 4, B = 3, C = 2, D = 1, F = 0. If there are +/-, there are minor variations based on whether +/- is worth +0.3/-0.3 versus +0.33/-0.33, and whether A+ earns the + bonus or not.