I can confirm that.
While launching gksu gdmsetup, stracing gdmsetup lock on:
futex(0x811d460, FUTEX_WAIT, 2, NULL
I wonder if it is somehow related to
http://bugzilla.gnome.org/show_bug.cgi?id=352263
** Changed in: gdm (Ubuntu)
Status: Unconfirmed => Confirmed
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gdmsetup doesn't open
https://launchpad.net/bugs/78475