When someone has taken too much of a drug Ghb and it makes them sleep (go on concious)
They normally go in and out of consciousness and its this that is them going into a g hole. Probably derived from K hole.
Having ingested enough GHB to put one in a trance-like, semi-aquatic state of disillusioned unresponsiveness as if one had fallen into a hole and could not get out.
Johnny had fed the hooker so much G that she was fishing out on the floor, deep inside a G-hole.