Not being funny Mate but this is your 7th Golf GTI and you don't know the difference between the 8v and the 16v ?? Also what checks did you perform when buying the car ?? Only asking as you don't know what engine is has and then you did not check for overheating etc. On the plus side even if the headgasket is gone it's not that expensive to fix but I would begin with the basic tests first like thermostat, temp sender or check for air locks.