Replacing KDM with GDM
From MEPIS Documentation Wiki
GDM login manager
apt-get install gdm
When prompted, select gdm to be the default display manager.
Removal of KDE
If you want Gnome to be your sole desktop environment, you will probably want to remove KDE because of its size. See KDE4 removal for instructions. Make sure you know which Gnome apps you are going to install first,
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:
(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."