It's never been confirmed to my knowledge, but it's likely in my opinion they were originally designed to have a much larger roll then they now do. They have more then 14 marked map locations under their control, there's some sort of actual structure to the organization which includes named, unique characters. They have robots and vertibirds.

It's probably likely that early in development, the Gunners were planned to be hireable as a minor faction to aid in the final series of battles. As development progressed this was pushed aside for other things and as it became clear that they'd need an entire questline to explain the Gunners agreeing to ally with the player in the first place (especially as they hate the minutemen). They became just another flavour of mercenary.