2004 INFINITY G35X SEDAN. TRANSMISSION LIGHT CAME ON AND CAR WON'T MOVE IN GEAR. POOL OF TRANSMISSION FLUID UNDER CAR. WHERE WOULD A LARGE AND SUDDEN LEAK COME FROM? DRAIN PLUG, SEAL, ETC.

There are a multitude of places that it could leak from. Being a 2004, I'd be suspicious of the trans. cooling lines which run from the trans to the radiator. Maybe, you hit something that flew up and damage a line? Of course, the front pump seal can dump fluid in a heart-beat. You'll need to be more specific as to where the fluid is leaking. Front of car, amid-ship, ect.
Loren
SW Washington