We get a description of Gatsby in chapter 3. Keep in mind that Nick, who is the narrator, has heard about this amazing guy, Gatsby, but has no clue who he is. He has also seen all the pomp and circumstance that surrounds this man, so it is hard for Nick to even imagine what Gatsby would look like.
While Nick is acquainted with most of the rich people from the Eggs, none of the people he knows are as flamboyant or excessive when it comes to entertainment. Hence, this Gatsby guy would have been a total enigma for him.
For this reason, he doesn't realize that the man "about his same age" that is sitting near him is actually Gatsby himself. Once identified, Nick takes a closer look and provides us with a more detailed description.
He first describes Gatsby's smile as "rare" and
with a quality of eternal reassurance in it.
Nick emphasizes on how odd it would be to know anyone in his society, or in life in general, with such a reassuring smile. Nick also explains that Jay would concentrate on his audience "with... irresistible prejudice," which could allude to someone who has a deep glance that can keep your attention.
Aside from the intense energy that Jay transmits in the way he treats people, Nick notes that Jay would assure you that you gave him the exact impression that you want to convey. This means that Jay is the sort of person who would do anything to make you feel safe and comfortable in his presence.
However, at some point, Nick gets off this momentary "trance" that Gatsby seemed to produce in him and focuses on the physical traits of Gatsby. This is when he realizes that Gatsby's behavior, when compared to his actual looks, betrays a degree of falseness, or perhaps theatrics, lurking beneath all that huff and puff of kindness.
This is exactly what keeps Nick so curious about this otherwise "ruddy" looking guy.
I was looking at an elegant young rough-neck, a year or two over thirty, whose elaborate formality of speech just missed being absurd.
Therefore, Nick takes a step back from admiring Gatsby and takes a look at the obvious. Jay Gatsby
is just "a rough-neck" that is perhaps too young to be talking the way he talks or acting the way he acts.
We also learn another physical description of Jay Gatsby:
His tanned skin was drawn attractively tight on his face and his short hair looked as though it were trimmed every day.
Therefore, what we learn is that Gatsby is a seemingly attractive and certainly charming young man. By the description of "rough-neck" we also learn that he may not possess that very sophisticated look that is expected of a man who has so much money.
The rest of the descriptions we get about Jay occur throughout the novel and are mainly related to his history and character rather than his physical traits.