If you want to do that at LA level then it takes you a lifetime to go around and count how many students have GPA>3 and GPA<3 then divide numbers to find percentage or proportion of GPA>3.

There are many statistical method to perform such tasks efficiently. For example, you randomly find 35 students and ask their GPA. Then find the mean and standard deviation and use a probability distribution like normal, t-dist,....to estimate prob(GPA>3).