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.