Confirmed in gdm 2.18.0-0ubuntu4.
/var/log/auth.log provides insight. I tested this with a user account named peter. Results below on attempted GDM login...
Apr 9 23:59:04 khermans-laptop gdm[9757]: (pam_securetty) access denied: tty ':20' is not secure ! Apr 9 23:59:04 khermans-laptop gdm[9757]: (pam_securetty) access denied: tty ':20' is not secure ! Apr 9 23:59:04 khermans-laptop gdm[9757]: (pam_unix) authentication failure; logname= uid=0 euid=0 tty=:20 ruser= rhost= user=peter
Confirmed in gdm 2.18.0-0ubuntu4.
/var/log/auth.log provides insight. I tested this with a user account named peter. Results below on attempted GDM login...
Apr 9 23:59:04 khermans-laptop gdm[9757]: (pam_securetty) access denied: tty ':20' is not secure !
Apr 9 23:59:04 khermans-laptop gdm[9757]: (pam_securetty) access denied: tty ':20' is not secure !
Apr 9 23:59:04 khermans-laptop gdm[9757]: (pam_unix) authentication failure; logname= uid=0 euid=0 tty=:20 ruser= rhost= user=peter