I recently picked up Shamal, my first foray into the world of Gong. It's good, a bit jazzy, I'll bet one of your three is the place to start, though. What do you recommend as the quintessential disk?
Paul
Hey Paul,
The group make-up on Shamal was a different Gong.  Yes, Shamal is quite jazzy, Steve Hillage plays only on 2 songs and then leaves the group, Daevid Allen (the group's founder) had left the group.  
As for a quintessential disk, I'd recommend  Gong's 
You this was the 3rd volume of the Radio Gnome trilogy.  

After the You lp the group moved in a different  direction under Pierre Moerlen the percussionist who joined Gong on Angel's Egg (part 2 of the trilogy).  Eventually the name was changed to  Pierre Moerlen's Gong, since there were a few other offshoots of the band.  
cheers,  Jim