You can use the touratech and other type of bracket for securing the GPS unit (you can even use the Garmin one that comes with the unit).
To mount the bracket/GPS to the bike like in the pics above, you can get the crossbar and mounting units from
http://www.gadgetguy.net. somewhat pricey but VERY sturdy and lots of options for monting.
others exist like the RAM units (that can attach to the master cylinder clamps on the bars).
http://www.ram-mount.com/
HTH,