One way to investigate the question of how narrative technique is used to explore psychological experiences is to look at the sections of The Great Gatsby that feature an impressionistic style (a flow of specific, observed details) and which present a somewhat unmediated narration wherein Nick reports his observations but not with much (or any) meaningful reflection.
An example of a section that uses the narrative strategy of impressionistic flow is found in Chapter 2, where Tom takes Nick along to a party with Myrtle and her friends in the city. For a large portion of the chapter, Nick reports on the events and people at the party as if he is watching from a distance. He has very little agency in this section and instead is a passive observer.
"I was so excited that when I got into a taxi with him I didn't hardly know I wasn't getting into a subway train. All I kept thinking about, over and over, was 'You can't live forever; you can't live forever.'"
Nick's lack of control over the situation is conveyed through the style of the prose as much as it is conveyed through the content of the prose. Thus his awareness in the chapter is expressed in bits and pieces, so to speak, and becomes a flow of drunken images and oblique remarks.
The party passage is characterized by copious amounts of dialogue that offer an impression of the people in the apartment (in a way that mimics impressionist painters who use suggestion and approximation to portray scenes as opposed to rendering scenes in complete detail). Impressionistic narrative sections in the novel help to demonstrate Nick's distance from the events he witnesses and also show that he feels incapable of truly or comprehensively understanding many of the things he sees (such as the behavior he finds at Gatsby's parties, the tawdriness of Tom's affair with Myrtle and her ilk, etc.).
The chapter ends with an example of an elided narrative wherein moments of action or details are skipped over and left out of the text.
"...I was standing beside his bed and he was sitting up between the sheets, clad in his underwear, with a great portfolio in his hands.
'Beauty and the Beast...Loneliness...Old Grocery Horse...Brook'n Bridge...'
Then I was lying half asleep int he cold lower level of the Pennsylvania Station, staring at the morning Tribune and waiting for the four o'clock train."
Using an elided narrative, the novel depicts Nick's drunken state and also his distance from the situation. He is adrift, as it were, and the use of a "gloss" in the narrative reflects this state.
These passages are almost like stream-of-consciousness narration, depicting a sequence of stimuli that Nick encounters and which often express a certain sense of alienation or of "being out of place" for our narrator.
Unable or unwilling to truly connect to the figures that populate the world he experiences this summer, Nick often slides into an impressionistic narrative that features objects/objective details and in doing so expresses an experience of alienation and isolation. Notably, Nick is like Gatsby in being out of place in the world he finds himself in. These passages express this notion through stylistic choices/narrative technique.
Fitzgerald is a master of narrative technique. He uses many of them to shape his whole story. So, the extent of narrative techniques is deep and pervasive. Let me give you a few examples.
The most obvious narrative technique is the voice of the novel. The story is told from the perspective of Nick. Therefore, it is written in the first person. That said, Nick says from the beginning that he is man who reserves judgment.
"I'm inclined to reserve all judgements, a habit that has opened up many curious natures to me and also made me the victim of not a few veteran bores"
This means two things. First, he is a more reliable narrator, because he absorbs what is going around him. Second, because people confide in him, he knows what is going on behind the scenes. In light of these point, he is close to an omniscient narrator. And he is by far the best judge of character in the book.
Second, another important narrative technique is duration. Duration examines the duration of the story and the length of the text. Some scenes are passed over quickly; others are expanded upon. When the latter happens, Fitzgerald draws attention. For example, instead of saying that Gatsby host many parties. The text lingers to show the decadence and fascination of these parties. Here is an example:
There was music from my neighbor's house through the summer nights. In his blue gardens men and girls came and went like moths among the whisperings and he champagne and the stars. At high tide in the afternoon I watched his guests diving from the tower of his raft, or taking the sun on the hot sand of his beach while his motor-boats slid the waters of the Sound, drawing aquaplanes over cataracts of foam. On week-ends his Rolls-Royce became an omnibus, bearing parties to and from the city between nine in the morning and long past midnight, while his station wagon scampered like a brisk yellow bug to meet all trains. And on Mondays eight servants, including an extra gardener, toiled all day with mops and scrubbing-brushes and hammers and garden-shears, repairing the ravages of the night before.
For more on this topic, consider focalization and repetition.