Hi there! The Great Gatsby is a great book, I've read it before ^^ usually on a test covering the book, there will be questions over events in the book because the events are really, really important! If your class is taking an "open-book" test, where you can refer back to the book, make sure to highlight, annotate, and put Post-It's on pages you want to refer back to.
If it's not an open-book test, expect more inference questions, main ideas and author's purpose, and broader details that don't require specific facts that can't be recalled without a book.
Also, teachers will ask for the ideas and themes within the book. Try to fully understand the whole situation about how Gatsby vied for Daisy's attention and that was his weakness throughout the book and eventually his downfall, and also how the events throughout the story match general themes.
Some teachers will go really crazy and ask for the littlest details, so it may be helpful to know a few! (ex. dates, locations, etc.)