Visit the following web site: http://www.foreigncredits.com/Resources/GPA-Calculator/Bulgaria
Fill in credits and grade (rating) for each subject in the respective boxes (add more rows if necessary).
For your information the Grading Scale in the web site is not completely correct, therefore for example if your rating is between 4.50 - 5.49, then consider it as Very Good and fill in 5 in the grade box.
Click on button "Calculate GPA" in order to calculate the GPA. The result of GPA calculated is on a 4.0 point scale, so please convert it in percentage.