Focalization is a literary theory that describes how a narrative is written. There are three divisions of focalization: internal, in which the story is revealed subjectively to the reader via a single character, external, wherein the narrator reacts and comments on events that he is witness to, that is the physical happenings, and zero focalisation is almost the same thing as having an omniscient (all-knowing) narrator.
Gatsby makes use of two of these focalisations: internal and external. It does not include zero focalisation.
The internal focalisation is provided exclusively by Nick, who tells the story of Gatsby. Here is an example of internal focalisation:
Of course I knew what they were referring to, but I wasn’t even vaguely engaged. The fact that gossip had published the banns was one of the reasons I had come East. You can’t stop going with an old friend on account of rumors, and on the other hand I had no intention of being rumored into marriage.
As readers, we get to know Nick a little bit here. He is not swayed by the opinions of others and he will not be strong-armed into anything he does not feel is appropriate for his own life. (It should be noted, however, that Nick is an unreliable narrator, meaning, you may not be able to trust everything the character says.)
And in this quote, we see external focalisation as Nick thinks about the sights and sounds of one of Gatsby's lavish parties:
In his blue gardens men and girls came and went like moths among the whisperings and the champagne and the stars.