Gatsby does allude to a sexual affair between Gatsby and Daisy, and deals with the outright subject of adultery with Tom and Myrtle. However, none of the sexuality is explicit or overdone.
Sexual? Yes. There are no really graphic descriptions of sex, but there references to immoral / extramarital sex are made,. What's more likely to offend these days, though, is the discussion of race by Tom, whose comments are quite racist.
“The Great Gatsby”
D---- mentioned 8 times
B---- mentioned 3 times only once about a dog
7 times whiskey mentioned
8 times champagne mentioned
2 times gin mentioned
5 times ale mentioned
4 times liquor mentioned
7 times cocktails mentioned
Drinking mentioned 15 other times all in a time when it was illegal
12 times Cigar/Cigarettes mentioned
7 other times Smoking mentioned
Over 20 times Deity name used in vain
Degradation of women and other people groups
Unmarried intimate physical relations written about and not viewed negatively
Adulterous relations written about and not viewed negatively
Many other times throughout very loose morals are written about and taught.
All in all I wouldn’t recommend this book at all to any child/person.