Actually the mount that comes pre-installed on the 2013 990 Baja is the the Touratech 'GPS Mount Adapter':
It provides the Touratech standard 12mm GPS mount bar.
It requires a Touratech GPS holder.
Just buy one of the Touratech GPS holders for your specific GPS and it will mount perfectly to the 12mm bar on the GPS Mount Adapter.
As an example, I have the Garmin Nuvi 550 GPS. I have the Touratech GPS Mount Adapter (what came on your bike from the factory). I bought the Touratech GPS holder for my Nuvi 550 and it mounts to the 12mm bar of the Touratech GPS Mount Adapter. Here is a pic of it Mounted: