We have 630 questions with the tag comics. These cover all sorts of comics.
We have questions about comic strips:
We also have, of course, lots of questions about mainstream comic books, which comprise the majority of these questions. For example, the most recent open question:
And we have questions about graphic novels.
A search of the tag comics with the keywords "graphic novel" brings up 24 results. A search for the keywords alone brings up 52 results, likely indicating that the majority of questions about graphic novels are not tagged as comics.
Some sources suggest that graphic novels are separate from comics. For example, here graphic novels are distinguished mainly by being long-form, or standalone stories. This writer takes a similar view, although they also mention aspects of the plot that might be too vague for our purposes.
On the other hand, some people see "graphic novel" as simply a pretentious way of referring to comic books of "quality," a tendency mentioned in some of the answers here.
Note the tag wiki description for comics, as it stands right now:
Comics is a graphic medium in which images convey a sequential narrative. The term could include 'graphic-novels'.
This seems to be more a suggestion of a synonym than anything else, but it does put forth the view that the two terms are equivalent.
My interpretation of the term "graphic novel," which I think is fairly close to the mainstream, is basically the same as the first one mentioned: a long-form or standalone graphic work. However, I think graphic novels are a subcategory of comic books, and thus any graphic-novel would be a example of comics.
I see three options: