I can confirm the same behaviour (on two different machines) with accounts from ldap and selecting Gnome in lightdm.
On every reboot regardless of the selection in lightdm a Unity (default) session is launched. On subseqent logins the requested Gnome session is launchen.
In lightdm.log
[+42.89s] DEBUG: Greeter requests session ubuntu
[+42.89s] DEBUG: Using session ubuntu
[+43.04s] DEBUG: Stopping greeter
Also in the greeter if I select Other..., select Gnome, type my username and press enter, then the selection is reverted to Unity in the password dialog.
I can confirm the same behaviour (on two different machines) with accounts from ldap and selecting Gnome in lightdm.
On every reboot regardless of the selection in lightdm a Unity (default) session is launched. On subseqent logins the requested Gnome session is launchen.
In lightdm.log
[+42.89s] DEBUG: Greeter requests session ubuntu
[+42.89s] DEBUG: Using session ubuntu
[+43.04s] DEBUG: Stopping greeter
[+144.73s] DEBUG: Greeter requests session gnome-shell
[+144.73s] DEBUG: Using session gnome-shell
[+144.74s] DEBUG: Stopping greeter
Also in the greeter if I select Other..., select Gnome, type my username and press enter, then the selection is reverted to Unity in the password dialog.