Well since I never said or implied that "acquiring a gamepad" is a "modification", you once again demonstrate that it is you who does not read (or cannot understand what you do read). But I'll be happy to explain. Gameplay that is specifically adapted for a gamepad controller includes adaptations to allow different parts of the controller pad to send control signals to the game (thereby allowing controlled user interaction). However, PCs do not normally include gamepad controllers. Therefore playing a game adapted for a controller, on a standard pc that has no gamepad controller, requires that gamepad signals must be modified so that they can be sent by a mouse or by a keyboard.
Adding a gamepad controller to a PC that does not have one would qualify as modifying the pc, btw. Since you probably don't understand this either, I'll try to explain it to you, as follows. A standard PC doesn't originally include a gamepad controller. When a gamepad controller is added to the PC. the PC is different than it was before the gamepad controller was added. Therefore the PC was "modified", as compared to its original non-gamepad-controller condition.