That is on our itinerary for this summer. This is the information that I have about it.
"Guðrúnarlaug is open to public and there is no entrance fee, you just show up, take off your clothes in a small changing room above the hot-tub and get into the tub. It is warm and lovely.There is a hotel in the Hotel Edda chain almost next to the hot-tub. It is lovely staying there and visiting the hot-tub late at night or early in the morning, when one can be there alone.
To get to this particular hot-tub turn from ring-road 1 onto road 60 for Búðardalur village. Drive for 16 km and turn left on side-road 589. Drive for 3 km on a paved road to Hotel Edda. Up in a hill immediately there is Guðrúnarlaug hot-tub."