See the article on power to connect the GPS.
The most viewable place is above the TFT screen where the “Remove for GPS Mount” is. There is a part from KTM (p/n 63512992044, €58) but it is moving around quite a bit.
In a pinch you can drill the part on the bike to put a RAM ball. This has been reported to be weak as the plastic part is not designed for it, and it causes vibration with large GPS. Innovative reinforcements have been implemented (back plate, epoxy glue filling…). But the tabs connecting to the bike are definitely not designed for this kind of load, if you do something like this do tether your GPS for safety.
Or you can print a support if you have access to a 3D printer, or pay to get it printed. Check the page dedicated to self printing your GPS mount. Use with a B-Sized RAM ball on an AMPS pattern (RAM-B-347U), a short RAM Arm (RAM-B-201U-A) to a RAM ball fixed on the back of your device (Which for a Garmin device mount is probably another RAM-B-347U). But to tuck the GPS as much as possible a combination of a RAM ball on the GPS and a socket on the mount makes for the best clearance. The socket is not made by RAM, but can be found at various places online.
MotoMinded took this 3D model, improved it, and is printing it in a sturdy material. The result is for sale for $22.
There is another 3D printable model, only at shapeways (aka, you can’t print it yourself). It is different and doesn’t seem to require any other hardware (note: it is in two parts, you have to buy both, and it’s unclear what hardware to use to link both parts. Let me know if you know more about this mount). Also, this is also designed with tabs which are a possible point of failure.
SW Motech now have a mount available, priced around €60. This projects very far back, over the TFT screen, the device can be mounted directly to the mount. It seems this setup is not very solid, and it wobbles a bit (video)
Givi has released a mount specific to the 790, putting a bar high up to fasten your devices to. Reference Givi FB 7710. There is no images in the product description on Givi’s page, but an ebay auction showed a couple of pics of the manual, see below. Around €40. This mount will only function if the original plastic part is present.
The Givi mount is hackable, one reader (hi Anders!) installed it rotated 180º to get closer to the screen (and higher), and reports that it can be drilled to eventually change the position of the bar. See pic below.
There is a handlebar mount from the 790 Duke that fits (P/n 64112992033, €40). But this put the GPS very low at a place where it is hard to read while riding.
On the handlebar there are other alternatives like the MotoMinded Stoutmount which is offering a little bit more flexibility in GPS placement.