Gatsby's dropping out of college was an important moment in this life. We find out that he attended a small college in Minnesota, St. Olaf's college (it's a real college, if you are wondering). Because Gatsby grew up poor, he had to work his way through college. He was a janitor to pay his tuition.
After two weeks he could not bear working because of the humiliation. So, he left and worked for a man whom he met as a fisherman on Lake Superior. This man, Dan Cody, took him under his wing. Cody was a self-made millionaire.
Cody made Gatsby his personal assistant. On their subsequent voyages to the West Indies and the Barbary Coast, Gatsby became passionate about wealth and getting ahead.
From this perspective, Gatsby's leaving college made all the difference in his life. From that point, his hunger for wealth, which was borne out of a desire to win Daisy, consumed his life.