DISPLAY=:20

** (gdmXnestchooser:18481): WARNING **: Can't exec, trying Xnest
Unrecognized option: 
use: X [:<display>] [option]
-a #                   mouse acceleration (pixels)
-ac                    disable access control restrictions
-audit int             set audit trail level
-auth file             select authorization file
-br                    create root window with black background
+bs                    enable any backing store support
-bs                    disable any backing store support
-c                     turns off key-click
c #                    key-click volume (0-100)
-cc int                default color visual class
-co file               color database file
-core                  generate core dump on fatal error
-dpi int               screen resolution in dots per inch
dpms                   enables VESA DPMS monitor control
-dpms                  disables VESA DPMS monitor control
-deferglyphs [none|all|16] defer loading of [no|all|16-bit] glyphs
-f #                   bell base (0-100)
-fc string             cursor font
-fn string             default font name
-fp string             default font path
-help                  prints message with these options
-I                     ignore all remaining arguments
-ld int                limit data space to N Kb
-lf int                limit number of open files to N
-ls int                limit stack space to N Kb
-nolock                disable the locking mechanism
-logo                  enable logo in screen saver
nologo                 disable logo in screen saver
-nolisten string       don't listen on protocol
-noreset               don't reset after last client exists
-reset                 reset after last client exists
-p #                   screen-saver pattern duration (minutes)
-pn                    accept failure to listen on all ports
-nopn                  reject failure to listen on all ports
-r                     turns off auto-repeat
r                      turns on auto-repeat 
-render [default|mono|gray|color] set render color alloc policy
-s #                   screen-saver timeout (minutes)
-sp file               security policy file
-su                    disable any save under support
-t #                   mouse threshold (pixels)
-terminate             terminate at server reset
-to #                  connection time out
-tst                   disable testing extensions
ttyxx                  server started from init on /dev/ttyxx
v                      video blanking for screen-saver
-v                     screen-saver without video blanking
-wm                    WhenMapped default backing-store
-wr                    create root window with white background
-x string              loads named extension at init time 
-maxbigreqsize         set maximal bigrequest size 
+xinerama              Enable XINERAMA extension
-xinerama              Disable XINERAMA extension
-dumbSched             Disable smart scheduling, enable old behavior
-schedInterval int     Set scheduler interval in msec
+extension name        Enable extension
-extension name        Disable extension
-query host-name       contact named host for XDMCP
-broadcast             broadcast for XDMCP
-multicast [addr [hops]] IPv6 multicast for XDMCP
-indirect host-name    contact named host for indirect XDMCP
-port port-num         UDP port number to send messages to
-from local-address    specify the local address to connect from
-once                  Terminate server after one session
-class display-class   specify display class to send in manage
-cookie xdm-auth-bits  specify the magic cookie for XDMCP
-displayID display-id  manufacturer display ID for request
-kb                    disable the X Keyboard Extension
+kb                    enable the X Keyboard Extension
[+-]accessx [ timeout [ timeout_mask [ feedback [ options_mask] ] ] ]
                       enable/disable accessx key sequences
-ardelay               set XKB autorepeat delay
-arinterval            set XKB autorepeat interval
-display string        display name of the real server
-sync                  sinchronize with the real server
-full                  utilize full regeneration
-class string          default visual class
-depth int             default depth
-sss                   use software screen saver
-geometry WxH+X+Y      window size and position
-bw int                window border width
-name string           window name
-scrns int             number of screens to generate
-install               instal colormaps directly

Fatal server error:
Unrecognized option: