Gatsby's Rolls-Royce figures prominently in the novel. Nick first describes it as "gorgeous" with a horn that plays a three-note melody. His description then becomes more detailed:
It was a rich cream color, bright with nickel, swollen here and there in its monstrous length . . . and terraced with a labyrinth of windshields that mirrored a dozen suns.
The extravagance of Gatsby's car represents his enormous wealth. However, it suggests not the muted elegance of "old money," but instead the lavish, gaudy excess of "new money." Gatsby's car symbolizes his place in society; he has money, but he will never be accepted in Daisy's world of old family names and inherited wealth. Tom alludes to this distinction when he refers to Gatsby's car as a "circus wagon."
Gatsby's car is very much a symbol of the man himself. Nick, indeed, first describes it as "gorgeous," just like Gatsby appears to others: well-dressed, well-spoken, well-educated. He appears to be perfect, just like his car.
However, Nick goes on to describe it as
[...] swollen here and there in its monstrous length with triumphant hat-boxes and supper-boxes and tool-boxes, and terraced with a labyrinth of wind-shields that mirrored a dozen suns. Sitting down behind many layers of glass in a sort of green leather conservatory, we started to town.
Words like "swollen" and "monstrous" have quite negative connotations, especially to describe such a "gorgeous" vehicle, alerting us to the fact that something more is going on here. "Swollen" is often used in connection to some kind of infection or illness, and "monstrous" connotes something grotesque, deformed. This might lead us to imagine someone who is puffed up, someone who has made something of themselves that is completely different from who they really are. Gatsby himself has become larger and stranger by his acquisition of all the material goods that seem to swell his car and draw attention to his giant size.
However, the labyrinthine, multi-layered glass seems like so many beautiful ways to distract from something or hide it altogether. Instead of clearly revealing the person within, they mirror "a dozen suns," protecting the identity within the labyrinth. Just as the car's windshields hide its driver, so does Gatsby's elaborate persona hide the person he really is.
The best way to answer the question is to give the description of it. Here is what the text says:
Gatsby's car was a ''rich cream color, bright with nickel, swollen here and there in its monstrous length with triumphant hatboxes and supper-boxes and tool-boxes, and terraced with a labyrinth of windshields that mirrored a dozen suns'
Again, the text says:
''labyrinth of windshields that mirrored a dozen suns.''
Based on these descriptions, it is clear that the Rolls Royce is over the top, excessive, and meant to impress. The second quote show the glitz and glamor. All of this fits Gastby's personality and persona - all for show. The irony is that this type of wealth is tacky and so will never be accepted by the old money. It tries too hard. From this perspective, the car symbolizes that Gatsby will always be an outsider.
More importantly, the car symbolizes Gatsby's downfall, as the car will crash and kill Myrtle. In the end, Gatsby, for all this wealth, will come to ruin as well.