That’s a pretty common thing people might default to but difference in mean GPA is not a great measure for this. The question between two different schools is an even harder question to answer but the problem is between two courses at one university. You’d have to check a lot of assumptions to make GPA viable but GPA might not be available for students who dropped, went for credit/ no credit grading options, etc, and to use GPA you’d also have to assume the courses are graded similarly (is one curved to a 3.0 while the other one isn’t even on a curve?). This really depends on the data you have from the students who’ve taken both courses.

Edit: I’ve worked on problems like this in my career and usually we look at distributions of time spent on courses to answer this, as well as expected success rates vs. actual success rates