Perhaps YOU should have checked: http://www.google.com/search?q=%22Trisomy-g%22&sourceid=ie7&rls=com.microsoft:en-US:IE-ContextMenu&ie=&oe=&rlz=1I7ADRA_enUS475
Apparently in the 1950s and 1960s Trisomy 21 was sometimes called Trisomy G.
I have come across no source other than Sullivan or commentaries on Sullivan that claim that Tri-G is common medical slang for Down's Syndrome.
On the occasions when I have heard a medical professional reference it, they called it "Down's" - not "Tri-G."