What proof is there is the beginning of A Gathering of Old Men that proves Beau came after Charlie with a gun?
When Lou Dimes first sees the corpse of Beau Boutan, there is a shotgun in the weeds a couple of steps from the body. Providing that the gun was not planted there by someone else, and there is no reason to believe that such a thing occurred, this would seem to prove that Beau Boutan did indeed go after Charlie with a gun.
When Candy tells Gil that she is the one who shot his brother, Gil says, "You're lying, Candy...Beau never would have come after Charlie with a gun. A stick, a stalk of cane, but never with a gun" ("Thomas Vincent Sullivan - aka - Sully or T.V."). Yet when Lou Dimes first arrives at the scene of the crime, he says that Beau Boutan lay,
"his mouth and eyes still open, his face caked with sweat and dirt, his dark brown hair speckled with dry grass seeds...he was about thirty, roughly handsome, maybe a hundred and seventy-five pounds...he wore khaki pants and khaki shirt and cowboy boots...his straw hat, bottom side up, lay in the weeds a few feet over to my left...a shotgun lay in the weeds a couple of steps to my right".
The tractor Beau was allegedly riding when he went after Charlie was still running when Lou Dimes came upon the scene. From the looks of the evidence, it would appear that, contrary to what Gil believes, Beau had gone after Charlie with a gun ("Louis Alfred Dimoulin - aka - Lou Dimes").