The Gunners are just Talon Company from FO3 with a different name. Their only purpose is to shoot you, with no real narrative place or sense. "Why are you shooting me? Your bullets cost caps, and I'm not doing anything to you, or even near one of your strongholds!"
I tried setting up a slightly alternate FO4 playthrough over the weekend, where I went straight from 111 to Nuka-World (story version: I was kidnapped for the Gauntlet right after waking up). It was tough, but it was mostly doable until the fight against the electrified power armor. I may turn the difficulty way down for that fight, then turn it up after.