Violent games don't "cause" acts of violence, school shootings or otherwise. In their sheer volume & alongside tons of other violent media, they do contribute to a glorification & acceptance of violence as a method of conflict resolution on an interpersonal & international scale.
I think it’s a very significant difference. The Trump administration should be focusing on the availability of guns; the focus on games is an absurd smokescreen that, among other things, makes having more nuanced convos about media and violence difficult or impossible.