I am about to leave for a summer trip (3 weeks).  I have a friend watching over the house.  How freqent  does the GS need to be started if it is parked/not ridden.  Ive heard about too many problems about engine needing to be recarbed if the bike sits for too long.   Can I just have him turnon the engine to warm it up without actually taking it out. 

None...just let it sit.

If the battery is in order it should be fine. I left my bike for 2 months sitting one time and it was fine (kinda had to work at starting it but no problems.

+1

3 weeks is a none issue. She be fine and waiting patiently for you.

mine wont sit for more than a week without needing a jump  , i'll replace the battery when it leaves me stranded somewhere.    then and only then will i'll learn to be more proactive 
and the dame thing is only 3 months old with just over 650 miles... 

Yep, but it on a battery tender (you've already got one, right?), and that's should be all you need.