One main difference is the pacing. The film may have a faster pacing in some parts compared to the novel to keep the audience engaged. For example, some of Gatsby's backstory might be presented more quickly in the movie.
The visual aspect of the film adds a new dimension. In the novel, we have to imagine the grandeur of Gatsby's parties, but in the film, we see it all vividly. However, this can also be a difference in a sense that some of the mystery and allure that the written descriptions in the novel create might be lost in the more literal visual representation in the film.
In the novel, the narrative is more in - depth regarding the social commentary. The film, while it does touch on the social divide, might not be as elaborate. Also, some of the internal monologues from the novel are either shortened or presented differently in the film. For instance, Nick's thoughts and observations are sometimes more nuanced in the written form.