There are several places to park if you want to go to Gaustatoppen. Make sure that you choose a car park that you are allowed to use, otherwise you risk a fine or your vehicle being towed away.
The easiest hiking trail to Gaustatoppen starts at Stavsro. There is a small pay car park here with a limited number of spaces. From Rjukan take the road up towards Gausta. Instead of turning left towards Gaustablikk, take the Fv651, and follow the signs for Gaustatoppen.