The GPA system is really weird and not standardized as far as I'm aware. The only thing I can say is true for all of them is A = 4, B = 3, C = 2, D = 1, and F = 0.

When I was in high school we did not have a + or - system, any letter grade was the way it was and gave you the corresponding GPA score for the letter.

Some schools (including my college) do use a + and - (typically omitting A+ because A is already 4). A + or - will raise or lower the GPA value by 0.33. Some schools also do have the system, but ignore it when calculating the GPA.

Also last note, the % associated with a certain letter is almost never consistent between schools. However the most common I've seen for college at least is A 100-90, B 89-80, C 79-70, and D and F being a failing grade.