I'll add a few things as well, concerning narration in The Great Gatsby.
First, almost all fiction is told in the past tense. Very little fiction uses present tense and tries to make readers pretend that the story is occurring simultaneously with their reading of it. It just doesn't work well.
Telling a story in the past tense, after the events have occurred, doesn't make for a story told in flashback. It makes for a story told in past tense. Nick's narration is no different from most other fiction in this respect. He openly talks about the fact that he is writing about the events after they've occurred, but that doesn't make the story flashback.
A flashback reveals occurrences previous to the present in the novel. When Nick describes Gatsby's pursuit of Daisy five years before the novel opens, he is using flashback. The entire novel is not flashback, however.
To add to what you know about the first-person point of view, Nick is an unreliable narrator. That is always a possibility with any narrator, but especially with a first-person narrator.
Nick makes a point to inform the reader at the beginning of the novel that he is objective and doesn't judge people. He uses an anecdote to explain to the reader that his father taught him to not judge people, because others didn't necessarily have all the advantages that he had. Nick emphasizes that he always tries not to judge people.
Of course, this shows that he often judges people. You don't have to excuse people for their backgrounds if you don't judge them negatively to begin with. And you don't have to concentrate on not judging people, if you don't judge people.
And Nick often judges people. He condemns Tom immediately, the first time he describes him, going back to his days at Yale with him. He also judges Jordan when they first meet. He makes value judgments throughout the novel.
I would just add to the first answer, that Fitzgerald uses first person flashback. While Nick does narrate the novel in first person, it is important to note that he does so in flashback. This type of narration tends to confuse readers because at points in the novel, Nick provides information that he learned from earlier in his relationships with Jordan and Gatsby (especially about Gatsby's beginning as James Gatz). If you can keep in mind that Nick is narrating the novel after all of the events occur and not in real time, that is extremely helpful. It is also significant to Fitzgerald's theme of the East's corrupting the innocent Midwest (Nick's telling of Gatsby's story and his description of Tom Buchanan demonstrate the loss of his idealism and optimism).
This novel is narrated in the first person, or from the first person point of view. The narrator always refers to himself as "I."
The narrator in this story is Nick Carraway. He is a young man who is distantly related to Daisy Buchanan and who knows her husband, Tom, from the time that they were in college together. He happens (by chance) to rent a home next to Jay Gatsby's mansion. This is how he comes to be in position to meet all the major characters and have insights into their actions and character traits.