Gdm, as a complete rewrite of xdm, uses its own configuration file, /etc/X11/gdm/gdm.conf. You should locate the definitions of the local X servers and add additional X servers for the number of cards you have.
SuSE uses:
/etc/X11/XF86Config
/etc/opt/gnome2/gdm/gdm.conf for gdm2
Debian uses:
/etc/gdm/gdm.conf for gdm
/etc/X11/gdm/ is a symbolic link to /etc/gdm/, so you can use both paths.
Red Hat and Gentoo use:
/etc/X11/XF86Config
Modifications:
.......
[servers]
# These are the standard servers. You can add as many you want here
# and they will always be started. Each line must start with a unique
# number and that will be the display number of that server. Usually just
# the 0 server is used.
0=Standard
1=Second
.......
Here, you'll have only 0=Standard. For each additional X server you should add a definition, like here, for 1=Second.
..............................
# Definition of the standard X server.
[server-Standard]
name=Standard server
command=/usr/X11R6/bin/X0 :0 -deferglyphs 16 -ac vt7
flexible=true
[server-Second]
name=Second server
command=/usr/X11R6/bin/X1 :1 -deferglyphs 16 -xf86config /etc/X11/XF86Config-4.[your card] vt17
flexible=true
..............................
And here, the exact definition of the command line for starting the X server,
very similar to the definitions in /etc/X11/xdm/Xservers
(the definition of [server-Second] is the additional one).
If you use a single XFree configuration file:
.............................. # Definition of the standard X server. [server-Standard] name=Standard server command=/usr/X11R6/bin/X0 :0 -layout first-Xserver -deferglyphs 16 -ac vt7 flexible=true [server-Second] name=Second server command=/usr/X11R6/bin/X1 :1 -layout second-Xserver -deferglyphs 16 vt17 flexible=true ..............................
Under older versions of Backstreet Ruby (released before 7. Oct 2003) each console is represented by 8 tty's, so you should use:
.............................. name=Standard server command=/usr/X11R6/bin/X0 :0 -deferglyphs 16 -ac vt7 .............................. name=Second server command=/usr/X11R6/bin/X1 :1 -deferglyphs 16 -xf86config /etc/X11/XF86Config-4.[your card] vt9 ..............................
If you use a single XFree configuration file:
.............................. name=Standard server command=/usr/X11R6/bin/X0 :0 -layout first-Xserver -deferglyphs 16 -ac vt7 ...... name=Second server command=/usr/X11R6/bin/X1 :1 -layout second-Xserver -deferglyphs 16 vt9 ..............................