It is pretty simple but each institute have their own way of converting percentages to grade point averages.
To start with the pretty simple way is to divide your percentage by 10( for a GPA on the scale 1-10) or dividing it by 20 ( for a GPA on the scale of 1 to 5).
Well, You get the point, If my percentage is 80, then My GPA will be 8 and 4 respectively.If your institution has nowhere mentioned how to convert your percentage to gpa or vice versa, you are allowed to follow this method when applying for other colleges/job etc.
But, Normally, These days colleges mention how to convert gpa to percentage and vice versa. So, you strictly have to follow that method when applying for a job/higher education etc.