The thing that makes the large quantity of American gun owners potentially useful in that sort of scenario is not that they possess guns. That matters a little, but usually most of the equipment a resistance uses is captured and/or supplied by allies. The thing that would be useful is these individuals' skills with firearms. It's theoretically kind of an alternate route to a similar (but lower) background proficiency level that some countries achieve with mandatory military service.

Note however that America is not only not unique in having that background proficiency -- but unlike mandatory military service, this approach has not really been tested. It's far from a certain proposition either way.