Replacing KDM with GDM
From MEPIS Documentation Wiki
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GDM login manager
If you prefer to use GDM (GNOME Display Manager) instead of KDM you can install the package gdm through Synaptic or with this simple apt-get command as root in a terminal:
apt-get install gdm
(This will take < 10 minutes to download over a dial-up connection.)
When prompted, select gdm to be the default display manager.
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GNOME desktop environment
If you are after a full GNOME desktop environment (not just the login manager GDM) then use Synaptic or apt-get on the command line to install these two packages:
gnome gnome-desktop-environment
(this will take a while to download/install)
Use Synaptic to install any other GNOME packages that you would like. Browse the GNOME packages available in Synaptic by selecting "Sections" (bottom left) then select "GNOME Desktop Environment."

