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I have it working but:
-I still don't know how gp-bikes names or selects or whatever the monitors.
-For third person playing isn't the best but also not the worst.  I can't see the whole bike.  GPBikes could have a FOV setting specific for this camera.
  Correction: It has a distance setting but it does not have enough range.
-No real support for different monitor sizes.  Height and Width settings for non main monitor work funny.  Side monitors size and bezel correction calculation missleading.

For first person playing is fine, everything is much bigger.
Vertical fov can get quite small, and under braking with head movement horizon goes away.

My setup is:
Position: Left - Center - Right
Windows ID: 3 - 1(main) - 2
24" 1920x1080 - 29" 2560*1080 - 24" 1920x1080

core.ini
[display]
monitor=1
setup=monitors.cfg

1 for main one.  Here I can also put 0.  0 seems to mean default monitor or main.

monitors.cfg
primary
{
   screen_distance = 500
   screen_width = 675
   screen_height = 283
   width = 688
   height = 313
}
global
{
   angle_x = 60
   angle_y = 0
}
monitor0
{
   display = 1
   res_x = 1920
   res_y = 1080
   multisample = 4
   bpp = 32
   width = 666
   height = 298
   pos_x = -1
   pos_y = 0
   shift_x = 0
   shift_y = 0
   shift_z = 0
   angle_x = 0
   angle_y = 0   
}
monitor1
{
   display = 2
   res_x = 1920
   res_y = 1080
   multisample = 4
   bpp = 32
   width = 666
   height = 298
   pos_x = 1
   pos_y = 0
   shift_x = 0
   shift_y = 0
   shift_z = 0
   angle_x = 0
   angle_y = 0   
}

Monitor naming or addressing or whatever - UFO
monitor0 - display=1 - pos_x=-1 - left
monitor1 - display=2 - pos_x= 1 - Right
I have no idea why monitor0 has to be display 1 and left, and monitor1 display 2 and right.

Monitor angle - OK
If they are aligned they are at 0 degrees.  500mm distance and 60 degrees give me 180 degrees FOV.

Monitor sizes - Buggy / Basic
Gpbikes works as if all monitors were of the same total size(width*height) and probably aspect ratio but lets you define sides screen size (height and width settings) and a custom resolution.
This means:
-bezels are calculated for main monitor (29" 21:9 2560x1080) with given parameters.

   screen_width = 675
   screen_height = 283
   width = 688
   height = 313
Calculated horizontal bezel each side is = 6.5mm

My side monitors are 24" 16:9 1920x1080, and their measures are:
   screen_width = 528
   screen_height = 298
   width = 550
   height = 327
Calculated horizontal bezel each side is = 11mm

But in order to have the right bezel compensation my settings must be:
   width = 688mm - 2*11mm = 666mm
   height = 298mm looks fine, its smaller than main height = 313.

Image on side monitors will be stretched but is not too important.
At least transition between perspectives is between monitors.  ACC, RF2, RR work even worst triple support and have them in center monitor.
Best support goes to AMS2 (direct multimonitor support without obscure config files or nvidia surround/AMD eyefinity) followed by AC with CM.