To get inputs from a gamepad you can include the input.h header file.

Most functions return a integer value that represents whether a button is being pressed or not

int buttonA;
buttonA = gamepad_button_a()

if(buttonA == 1)
{
  print_at(0, 0, "Button A pressed");
}

or

if(gamepad_button_a() == 1)
{
  print_at(0, 0, "Button A pressed");
}

Some functions require an adress input: such as gamepad_direction()

int directionX;
int directionY;

gamepad_direction(&directionX, &directionY);

if(directionX == 1)
{
  print_at(0, 0, "Right pressed");
}
else if(directionX == -1)
{
  print_at(0, 0, "Left pressed");
}

if(directionY == 1)
{
  print_at(0, 18, "Up pressed");
}
else if(directionY == -1)
{
  print_at(0, 18, "Down pressed");
}