I am positive the F800GS computer is programmed for a 17" rear wheel and a 21" front wheel. When you switch the front 21" for a much smaller 17" wheel, the smaller diameter of the wheel will in turn make it rotate considerably faster. This considerably faster spinning front wheel will confuse the ABS system. The computer in the F800GS is static, not dynamic. It doesn't "compensate", and it could be life threatening if you leave ABS on. I don't recommend it.
As an example: My buddy that has a 2007 Chevy Silverado just put larger tires on the rear and a bit smaller ones on the front to give it that "old school" look. Completely wigged out the ABS sensors, and his dashboard ABS alarm goes off constantly now. He called his Chevy dealer and they said it would be $200 to reprogram the computer to handle the tire size difference. If the far more advanced automobile computer control systems in a new Chevy can't handle a tire size change...the simple solid state computer module on the F800GS motorcycle definitely won't.
That aside, I do know that the F800GS won't dynamically handle a wheel size change.