With a bit a shame I realized I probably forgot to mention that GMPC now has a website (instead of just a wiki). I did link to it several times (because of the manual) but you might have missed it.
So please check it out:
http://gmpclient.org/
This site is made and maintained by Timo, who I cannot thank enough for his work. I hope to have the more important information available here. I am absolutely disgusted with how the (new)current wikipedia theme looks, and the ads make it even worse.