My bad I didn't completely clarify. The Gunners are the largest threat to the Commonwealth with the Institute gone. I'm unaware if they have any other expanded operations besides bases added in Nuka World. Just listed them for their expertise in battle, since they're an organized outfit of troops who fight as an effective unit capable of ambushing Recon squads from the Brotherhood.
I see them as the biggest threat in that district, but overall Eastern Front I'd say Super Mutants as they can use weapons, and even build their own armor making them extremely dangerous. Secondly I'd say Feral Ghouls as in large numbers whether by ambush , or just them having a elite squad beat by sheer numbers can prove devastating.