Good question. So many people don't know about either. I'll try my best to describe the difference. Graphic novels are loosely termed (by all accounts) and include comic related elements or illustrations that are central to the narrative of the story. The way in which they are used and created varies widely but the images help aid the imagination and guide the story.
Multimedia novels come in all shapes and sizes and are a much broader category as, effectively, they just need to be told using a number of media types. Because images are just one media type, any multimedia novels with just images would - I guess - be categorised as graphic novels. Usually multimedia novels just bring to life one element of the story through sound, imagery or both.
In the case of The Frequency Effect, it is the first story in the world to combine the experience at the cinema with a soundtrack, images, narration and so on. That is why is fits into a new 'Immersive Cinematic' category. At the same time, to provide an added twist, the story happens in real-life. It is an immersive story that can grow and give fans the opportunity to participate.