For the most part, Gatsby is straightforward. It's got some funny 1920s turns of phrase, like "ecstatic cahoots" (8.46), but you're not going to run into too many unfamiliar words.
But that doesn't mean the book is easy. Narrator Nick has a literary bent, so occasionally he lets loose with something like this:
His gorgeous pink rag of a suit made a bright spot of color against the white steps, and I thought of the night when I first came to his ancestral home, three months before. The lawn and drive had been crowded with the faces of those who guessed at his corruption – and he had stood on those steps, concealing his incorruptible dream, as he waved them good-by. (8.46)
Hard? Not exactly. The toughest word is "incorruptible," and we bet you've got that one. But the way he moves from the present to the past, and really-not-clear-at-all phrases like "concealing his incorruptible dream"—make this a paragraph that you're going to want to read slowly.
But, then again, why wouldn't you want to take your time over knockout sentences like that?