There are only two places in Iceland where you can observe Northern Gannets. One of them is Rauðinúpur, and the other is the rock Stóri Karl.
You can find Rauðinúpur almost at the very north of Iceland. If you want to visit this place, park your car at these coordinates (GPS: 66.506950, -16.502736) and then walk along the stone dam, following the water. It will take about 20 minutes on foot to reach the spot. Drive carefully!