The 'guest' user created by gdm-guest-session is a passwordless system user by design, and gksu fails when trying to prompt for a password. One way to make the error dialog appear might be that gksu checks whether the GDMSESSION environment variable equals 'guest-restricted', and skips the password prompting attempt if it does.
The 'guest' user created by gdm-guest-session is a passwordless system user by design, and gksu fails when trying to prompt for a password. One way to make the error dialog appear might be that gksu checks whether the GDMSESSION environment variable equals 'guest-restricted', and skips the password prompting attempt if it does.