The cache is located in a cute little wood near the rivulet Gíslalækur (Rivulet of Gísli) and the start of the trail Gíslagata (You guessed it! Trail of Gísli! He was probably a settler who lived here).
Here is a link to the hike: https://ferlir.is/gislagata-sandfellsvegur-svinaskardsvegur-3/
The wood is small but lovely, with a picnic table waiting for you! Nobody is ever here so you will have the place to yourself :)