Underweight by what measurement? Don't GPs usually go by BMI (which isn't exactly the most reliable thing anyway, although I'd imagine it's more accurate at lower weights since it's usually muscle mass that throws BMI measurements off) to determine this? The BMI calc on the NHS's own website claims you're a healthy weight. Even at 18 you'd be considered a healthy weight, the 'healthy weight' range at 18, considering your height is 65.4-88.4KG, this is information directly from the NHS themselves.
I'm not sure what your GP is doing, but my best guess is that he's seen that you're close to being considered underweight as far as BMI goes (the weight range shifts a bit from 17 to 18) and is trying to get you to gain weight. What I can't understand is why your GP has told you that you're underweight as opposed to saying you're close to being considered underweight.