GUI sudo not working?

I accidentally screwed up the groups for my main user by doing sudo usermod -G blah instead of sudo usermod -a -G blah. On realising this, I immediately rebooted into single user mode and added the groups adm dialout cdrom floppy fuse to the user's account. Everything seemed to be more or less back to normal and I could sudo as usual on the command line. However, the authentication dialog box that shows up in the GUI has stopped working for nearly all programs. The dialog shows up as normal, but will not accept the password. gksudo firefox from the command-line seems to work, as does Synaptic, leading me to believe that I'm merely missing a group that the other Ubuntu configuration/admin apps check for. Any solutions would be much appreciated.