not 3 letters of than a big bold WTF explains my feelings. I went through the non existing documentation of gdm2, tons of man pages, spend more than 5 hours of my life on this problem, reinstalled the fedora 4times... than just after I write this post and prepare to sleep, out of the blue ,automagically, a sessions dialog appears in the gdm.
It wasnt there yesterday, it wasnt there this mornin, maybe some bug maybe something else.
if anyone faces the same problem try reinstall gdm2 or the whole os. If all fails go with kdm or wdm, use the `yum install system-switch-displaymanager` if you are a fedora guy as well