It seems that a lot of GPU problems revolve around specific versions of drivers. Though AMD has their own support structure, you can often learn from information reported by others who fold.
This is the entire message that FAHcontrol show me when i open it:
"On client "local" 127.0.0.1 Option 'gpu-index' has no default and is not set.
iter should be a GtkTreeIter"
If you have run the Uninstaller from the Folding@home folder in the Applications folder, and deleted the FAHClient folder from /Library/Application Support/, then all files associated with the F@h client should have been removed. Both operations would require use of an admin account password.
If you have deleted everything, then by definition a fresh install would not have a config.xml file. At that point you should be prompted for the username, team and passkey by the client opening up Web Control. If it opens in Safari, change the URL from https to http and refresh.