iStudiez Pro was designed and is supposed to keep a track of your current classes, assignments, exams, etc., i.e. to operate with your actual data. 

Thus, there's no option to show several Overall GPAs (current & past) in the app.

On the other hand you may exclude your past grades from Overall GPA calculation.

Go to the Planner > Previous Semester > Course > Grades > uncheck the option to calculate the course grade.
This way you'll keep your past grades in the app, but they won't affect your current Overall GPA calculation.

We may offer you one more way out. 

Create iStudiez Pro data backup for your previous studies/semesters, delete those entries from the app and use it for your current study process only.
Keep that backup safe and restore it whenever you need to access that data.