You might try /r/Norse - it is home to many native speakers of Icelandic and there are others who have studied Old Norse intensely.

Just as it is spelled, of course! How else would you pronounce anything?

More seriously, it is pronounced, roughly, "Gnaw-thee-math-er", coming down harder on the 'g' than you would in English, and almost eliding the final 'e'.

Not sure about the vowels, but that funny-looking "d" character is pronounced like the th in "then" or "that" -- but NOT like the th in "thing" or "thick".

Gnaw-thee-math-er, as said by someone else already.