for the functionality which is mapped from other libraries, you'd be better off just using the original documentation. In this case, the glfwGetGamepadState is pretty well described here: https://www.glfw.org/docs/latest/input.html#gamepad
as for how to use it within Java, you just need to instantiate a ByteBuffer object, which will be filled with the state of the gamepad and create a new instance of GLFWGamepadState.
mDataByteBuffer = MemoryUtil.memAlloc(GLFWGamepadState.SIZEOF);
mGamepadState = new GLFWGamepadState(mDataByteBuffer);
You can then poll the gamepad state when you're handling input like this:
if(mGamepadState .buttons(GLFW.GLFW_GAMEPAD_BUTTON_X) == GLFW.GLFW_PRESS)
Not all controllers have a gamepad-mapping available, so you first need to check the availability of the mapping by calling glfwJoystickIsGamepad