The default display manager (a.k.a. login screen) of the GNOME desktop environment
GDM (short for GNOME Display Manager) is an open source graphical login manager specifically designed to be used as a display manager for the GNOME desktop environment. It is used in many well known Linux distributions. It’s comprised of a system service responsible for providing graphical log-ins, as well as for managing both local and remote displays, and the graphical login part that features a face browser, a language/session type selector and an optional logo.
Protects your computer from pesky people
There’s not much to say about a login manager, because it runs at boot time and you interact with it maybe a few times a day, when you login and when you lock your screen. But it’s there, it protects your computer from intruders or pesky people.
Helps you easily switch between multiple desktop sessions
In addition, the login manager usually allows a Linux user to switch between different desktop environment that are installed in the respective operating system. It also lets you to quickly login as a different user, use a guest account or the remote login function.
Looks very professional
When compared with other similar products, we can immediately notice that this project looks very professional, and provides users with state-of-the-art functionality. In our opinion, LightDM can be made to look even more professional than GDM, and provides some extra functionality.
Designed for GNOME
By default, GDM (GNOME Display Manager) is distributed as part of the GNOME desktop environment, but it can also be used as a standalone application that provides login capabilities for other desktop environments or Linux operating systems. However, GDM supports any Linux-based operating system and several desktop environments, including Xfce, Cinnamon, MATE, Openbox, Fluxbox, KDE, LXDE, and others.
We suggest GDM if you want a mature and full-featured login manager for your Linux distribution. But, if you operating system supports LightDM, we feel obliged to urge users to stick with it because it's much more powerful and provides state-of-the-art and modern functionality.