a. Create a new forum
b. SOP - anything and everything related to guns
c. No guns related OPs in GD. None at all as it's the easiest rule to enforce and the most fair to both sides, and the rest of DUers.

I perfectly understand there will be a lot of angry people on both sides, but this way both sides can post in the same place, fight as many battles as they are pleased, and leave the rest of DU to discuss other topics.

First group of new hosts can be selected by voting: ask both sides to nominate equal number of candidates in two separate threads. That should avoid any accusations of unfair treatment.