For everyone who plants to visit Iceland, and considers camping, have a look at campingcard.is. This card costs you about 150 bucks and gives you access to a wide range of camping locations. The locations are good, sometimes, especially when it's raining, the provided indoor accommodations are a bit crowded. Still - it's a nice deal. We did it for about two weeks and it was a nice experience.
Unfortunately campingcard doesn't provide GPX files, so I wrote a python script to extract the locations from campingcard.is and write them to a GPX file last year. Both, the files and the script are available on GitHub.
If you are searching for the CampingCard GPX files, then you also should have a look at https://safetravel.is/ for safety informations around Iceland.
Download CampingCard locations as GPX
I've hosted the extracted GPX files for 2017 and 2018 on my blog - you find them here. Please, use at own risk!
Have fun and keep safe!