Hard to make a diagnosis from the sketchy info but GTIs certainly aren't noted for their absorbtion of sharp bumps and potholes. That's the price we pay for the class leading chassis away from the city centre and onto proper roads.
Things that can make the car ride badly vary from worn bushes, over/under inflated tyres, knackered shocks and snapped springs.
The Golf is a lardy car and the roads are terrible in the UK so you're going to see wear and tear down below.
Can you give us some specifics of when the ride is at it's worst, mileage the car has covered, type of roads you use and speeds you drive at.........etc