Here's how I tested this fix:
Boot the iso into Install mode.
Click Quit.
Log out of GNOME.
Switch to a virtual terminal.
Log in as ubuntu-gnome.
sudo systemctl stop gdm
Make your changes (in this case, editing /usr/bin/ubiquity-dm)
sudo systemctl start ubiquity
Thanks fossfreedom for pointing out the fix.
Here's how I tested this fix: ubiquity- dm)
Boot the iso into Install mode.
Click Quit.
Log out of GNOME.
Switch to a virtual terminal.
Log in as ubuntu-gnome.
sudo systemctl stop gdm
Make your changes (in this case, editing /usr/bin/
sudo systemctl start ubiquity