After doing a quick Google search for "GPMG vehicle mount" or simply GPMG mount, (along with variations of C6 or M240 in lieu of GPMG) a few sites came up.
http://www.sageinternationalltd.com/si/mount/soft.html Although not the pictured type, it may be of interest to you.
http://www.manroy.com/html/gpmg_buffered_mount.html
The ones in use by the CF in the above pictures all have the common element of being buffered mounts (just like our sustained fire tripod).
The variations might indicate, and allow you a freedom in the choice of mount for your Iltis. Just wondering, do you have an M240 to mount with it?
I hope you'll post pictures of your finished project when it comes to fruition!
Hope this helped a bit.