While the letter grade to numerical contribution for a single subject is ordinal, the moment you compute a grade point average you already treated it as interval at that moment (otherwise you have no basis on which to assert that A+C = B+B).
General rule of thumb: if you can add it, it's quantitative. For example, a G.P.A. of 3.3 and a G.P.A. of 4.0 can be added together (3.3 + 4.0 = 7.3), so that means it's quantitative. While the letter grade to numerical contribution for a single subject is ordinal, the moment you compute a grade point average you already treated it as interval at that moment
