Graphic novels are fundamentally novels. They should have the structure of a novel with a fairly clear beginning, climax, and end. It should be evident what the primary conflict of the plot is. Some comics are episodic - parallel to short stories, rather than novels. They may have multiple conflicts of equal weight.
It gets tricky when deciding whether a graphic novel can have multiple volumes. Nowadays novels are published in one volume, but many classic novels were originally published over multiple volumes (ex: Jane Eyre and Emma were each published as three volumes). Other classic novels were published as serial novels (ex: Great Expectations and Twenty Thousand Leagues under the Seas). The internet is bringing back the serialized novel form. Many manga and manhwa are, in my mind, serial graphic novels. However, I've never seen anyone use the term.