TGG needs no introduction. I read it at O-level and now read it again, since Miranda is reading it for GCSE. Why is it good? It is well written, worldy-wise, and tells an interesting story of interesting times. Perhaps it helps to know the story; it makes early promises like there was something gorgeous about him, some heightened sensitivity to the promises of life easier to read in context.
It helps that a lot of it is about rich people. It's kinda nice to read about lovely women lying gracefully on chaise longue looking elegant. There's a certain connection to Proust about it all; the young narrator moving up into a stratum of wealth and privilege, and naturally this is a pleasant world to be drawn into reading about.
Is it a great novel? How could you rank it against, say, Heart of Darkness? Lower, I think. There something slight and fluffy about it, coming perhaps from the subject matter; it is just rather hard to take it all seriously, no matter how seriously it is all clearly intended. Gatsby's great obsession, Daisy, isn't really worth his attention. The book attempts to finesse this at the end, and turn it into Gatsby chasing his own past - a-la "a la Recherche" - but I don't think it will quite do. Somewhat less important: the book hints rather strongly that Gatsby's money is dishonestly arrived at. By implication, it suggests this isn't important: the whole society was rotten, Gatsby's hard-to-define fine qualities more than outweigh this; but it isn't clear we should agree.
But it is a very well written book. I say that as a reader of sci-fi, of course, much of it of rather low literary quality, so I'm hardly a good judge. I made a brief attempt to find something to prove how well written it is, but it would be pointless.