How do you get grade points on hunter story

You get grade points by doing missions. The better you do at a mission, the more Grade Points you get. These newly earned Grade Points unlock more missions, which you can do for more GP, which unlocks even more missions, etc.

Grade Points are determined by rank, and rank is determined by score. This is the rank list:

E - Fail(0GP)
D - Almost Fail(3GP)
C - Ok(5GP)
B - Better(8GP)
A - Even Better(10GP)
AA - Awesome(12GP)
AAA - Super(13GP)
S - Best Rank You Can Get.(15GP)