In GMPC's preferences, I'm connected to localhost port 6600 for MPD, and my music library is set to the correct location. When I go to the Databases tab on the left, it shows a folder labeled "/", probably meaning root, but I can't open that folder in GMPC. I have clicked the "update database" button. Any possible solutions?

I'm having the exact same problem. I just installed GMPC on Ubuntu 11.10, and am currently trying to fix that problem. First shot (editing /etc/mpd.conf) didn't work.
Any ideas?

Ok, after having broken and somehow fixed mpd it worked. I guess this is what did the trick in the end:

1. close gmpc, then stop mpd: open a terminal and type

sudo /etc/init.d/mpd stop

2. Edit the file /etc/mpd.conf (you probably have to be root for that as well): change the variable music_directory to your desired directory

3. restart mpd: in the terminal, type

sudo /etc/init.d/mpd start

Open gmpc again. Does it work?

EDIT:
I had trouble using stop / start as described in 1. and 3. - could not create pid-file or something similar. Solution:
In terminal, type

killall mpd

Then, in /etc/mpd.conf, change the pid_file to: /home/<your username>/.mpd/pid
Create the directory .mpd (e.g. in terminal: mkdir /home/<your username>/.mpd)

Then start mpd as in step 3.