I know there's the Socialist Rifle Association and Pink Pistols. (It's a LGBTQ gun group, is it wrong to assume they are leftist?)
I'm just trying to find examples of leftist groups that are pro gun. I am thinking about aligning with them and finding out if they write to their representatives the way we (we being presumably republican gun owners) do to fight gun control. I think it'd be more productive to send letters to democrats from democrats.

What are some other groups?

John Brown Gun Club and The Liberal Gun Club come to mind.

Black Guns Matter is not a leftist organization AFAIK