You can buy an add on bar from BMW for the GS, then you can mount the Satnav.
I found it on eBay:
Genuine BMW Motorrad GPS Sat Nav System Mount Support R1200GS 71607707369
There is one on eBay for £30 + postage eBay item number:253426874735
I found it on eBay:
Genuine BMW Motorrad GPS Sat Nav System Mount Support R1200GS 71607707369
There is one on eBay for £30 + postage eBay item number:253426874735