I've never done gamepad input, but the above suggestions look promising. I would also check out this page for a quick intro to inputs: https://docs.unity3d.com/Manual/ConventionalGameInput.html
A lot of the standard inputs are mapped to VR/gamepads by the SDK I believe. For example, Input.GetButtonDown("Escape") would respond to the escape button and back on a GearVR.
Input Manager is fine for your demo project, but if you want to support a BUNCH of controllers, Rewired or InControl are worth the money. However, someone nicely has posted the button mappings you need for getting the controller Oculus gave us (Steelseries Stratus XL) and that is for free!
Go into Edit-Project Settings->Input and add new array elements. Call one "Submit" for your button A on the controller. And in your code, Input.GetButtonDown("Submit") will return true if you press the A button. dpad and joysticks are a little different, but the internet has a bunch of info on them as well if you search.
