I think your idea is entirely possible–that the hill is named for its shape rather than for the presence of cannons. I would want to look at more maps to get a better look at the topography there.
The William’s Bridge crossing of the Bronx River, which is adjacent to Gun Hill, was written about before the Revolution. It was the boundary between Yonkers and the Manor of Fordham and it was named “Cowangongh” according to the 1669 deed between the Munsee and John Archer, who was the lord of the Manor of Fordham. That deed is interesting because it contains many of the Munsee place names for locations in our part of The Bronx. You can check that out here. I recently got a chance to look at a photograph of the original deed and it is amazing to see because the Native Americans signed it using symbols or pictographs instead of spelling out their names.
I think the Gun Hill would have been important because it overlooked this crossing of The Bronx River. It would be pretty funny if its name comes from its shape as opposed to the fighting that occurred there.