“They have all the guns” is a metaphor in the context of class warfare.
I mean that they have the means to employ force (usually through police, but not exclusively) in their interest as well as having the entire power of the state behind them (disproportionate wealth means they have disproportionate political influence which means they can lobby for laws to be adjusted in their favor. Even when the law seems just, it is rarely applied in the same way to wealthy people in practice).
Not to mention that they can and do buy influence over the media apparatus, controlling narratives and tricking the working class into acting against their own interests.
Within the framework of class conflict, those are the “guns”.