Nukes wouldn't come into play. Assuming that it's a leftie government that wants to take away guns, but Nukes are area weapons. They would have to nuke a lot of America to kill gunowners. During the cold war us and Russians aimed Nukes at each other's cities, not farms. So using a nuke against East Bumblefart and it's 1200 residents is stupid. Using Nukes to take out a major city "makes sense".
So the only folks that the lefties could use nukes on are.....their own cities. Not that they wouldn't be above nuking East Bumblefart. But doubtful as "Nukes are bad, M'kay."
But as RKL mentioned, they would have no problem rounding up people whom have "wrongthink" and putting them into camps to either re-educate or exterminate. We have many, many examples to point to as a guide to what the left is capable of doing to those with whom they disagree.