Is it a Gun Issue or a Mental Illness Issue or Both?
Not sure what you mean by gun issue. I interpret that as "gun access" issue. I think that the ease of access to weapons in USA may have contributed to that unfortunate event, but seems like mental health (I'm not talking about severe mental illness in this case) played a role. Learning a little about the background of that shooter, seems like he could have used some kind of support system. Unfortunately he found himself with no parents too early in life and completely alone growing into adulthood all by himself.
Then today I learned that many people around him and online did the right things. There were numerous reports to the police and online security "authorities" (reporting things online seems like a joke) about his postings and his behavior. The FBI did not follow up.
As always, just because they are several events that resulted in violent attacks like this one, doesn't mean that they are the result of the same issue. In this case several things went wrong, his past, his life, his mental health, the authorities not paying attention to multiple reports on his postings, activities and behavior, and seemingly easy access to weapons without a lot of questioning.
As for the weapons, I'm not completely sure. If he wanted to cause such harm and didn't have any weapons, he may have burned the school or who knows what else could he have done if he's set to kill and cause harm to other people. The means in this case, I think are just a catalyzer. There are larger issues to look at in this one.