The easiest method I have found through blueprints is to spawn and attach the weapon the same way hands are spawned and attached to the MotionControllerPawn. By default, the Pawn doesn’t have hands, they are spawned under his BeginPlay event. You can add an event to the the Pawns BeginPlay event that will call an event in each hand to Spawn and attach your gun to each hand.
Alternatively, rather than spawning the default MotionController, you can spawn one that is already set up with the gun. Hope this helps!