The best advice to be accurate with time and tense when describing events in a novel is to assume your audience knows nothing about the novel. So, you want to explain everything in order that your reader understands what happened, when it happened, and who was involved. You can use the past tense or the present tense; just be consistent. If you choose past tense, stay with it.
Example using the past: Gatsby met Daisy in Louisville. He went off to war and did not meet her again until five years later.
Example using the present: Gatsby meets Daisy for the first time while he is stationed in Louisville. He goes off to war, promising himself that he will return and marry Daisy. Five years later, Gatsby meets Daisy again at Nick's house.