Just a small note on this one, because I often see people mistakenly abbreviate it "GK": it should be parsed geir skogul, notgeirs kogul.
As for its pronounciation, r+s in Norse-based languages become "sh", even when separated by a space, so something close to "gish-kugl" is a far better approximation for it than "geer-sko-gool". If you're interested in hearing a native speaker of Icelandic and Norwegian speak it, this YouTube user says it while reciting part of the Völuspá, about 12 seconds in.