The current situation at Spiterstulen is that you must either camp at the campsite next to Spiterstulen and pay a fee, or camp at least 1 km away. While it's otherwise supposedly forbidden to camp within the 1 km zone, what you risk is paying a fine or fee equivalent to the camping fee at Spiterstulen.
Use a GPS to make sure you pitch you're tent more than 1 km away. The staff at SPiterstulen take "rounds" every now and then to collect fines or fees from those unfortunate enough to have camped well away from Spiterstulen, but still within the 1 km radius.
There are two English language guidebooks that cover Galdhopiggen:
A. Dyer et al's guidebook "Walks and Scrambles in Norway" ISBN 978-1904466253.
Bernhard Pollmann's "Norway South" also covers it, but is out-of-print and is more difficult to find (ISBN 978-3763348077).