You can use a Basic Universal Calculation to work out your GPA. To do this you assign each grade a value (A+=9, A=8, A-=7, B+=6, B=5, B-=4, C+=3, C=2, C-=1 (fail grades, including DNS (did not sit) and DNC (did not complete) should be recorded as a 0.)), calculate the total and then divide by the number of papers you have grades for. You can do this manually, or by using the tool below and putting 1 in as the points value for each paper. You will need to do this first for just your law papers to get your Law GPA, and then add in all your other papers to get your total GPA. This method assumes each course is worth the same number of points, which is not accurate, but is generally widely accepted.
For a more accurate GPA calculation, you need to take into account the points each paper is worth (which varies from paper to paper and university to university). You can do this using the tool below and putting in each paper, its points value, and the grade you were awarded. Again, first for just your law papers and then adding in your other papers for your total GPA.