Here's the text of what he said “Hillary wants to abolish, essentially abolish, the Second Amendment, by the way, and if she gets to pick, if she gets to pick her judges, nothing you can do folks, although the Second Amendment people maybe there is, I don’t know but, that will be a horrible day, if Hillary gets to put her judges…”
So the the set-up for the "joke" is "if she gets to pick her judges, nothing you can do folks" and the punchline of what you claim is a "joke" is the "...although the Second Amendment people maybe there is" (something they can do about their claim that Hillary is going to "essentially abolish  the Second Amendment)?
What could or would the Second Amendment people do that other Republicans (and presumably Democrats who also support Second Amendment rights) cannot do? There is no joke to get if the joke isn't a reference to the Second Amendment people taking extralegal measures against the presumptive POTUS Hillary Clinton. Everyone got Trump's suggestion.