I think the gunners are a cool unjoinable faction. Captain obvious, they are mercenaries for hire. From all the side quests, hollow tapes and characters models clearly they have some form of way to operate in very organized way and the tattoos on their face is a some form of a branding. I just think if they were joinable it would of added a nice way to role play a character in that group. Perhaps they could of been one of the 4 factions to take down the institute, for the right price of corse. It could of been like Skyrims dark brotherhood quest. You get an offer you just can’t refuse so the question is how to take them down so you can get payed very handsomely. Perhaps there was even a division for bounty hunting in their faction. There’s a lote of things I felt they could of done. Maybe even hinted that In time they could build up to pose a threat to even the brotherhood since they are so fascinated with pre war weapon technology to use in their own military. The only difference is the lack of brotherhood “morality”. Maybe even side quests to build their military up, both in man power and tech, Wich is another path it could of taken. The brotherhood is already built and the minute men feel more like one big neighborhood watch so it would of been cool building an army