A God Who Doesn't Say All He Knows

Etienne Gilson, Being and Some Philosophers, Pontifical Institute, 1952, p. 177:  "God knows essences, but He says existences, and He does not say all that He knows."

Here are the makings of a good examination question for a course in Thomism:  What is the Frenchman driving at?  Unpack the Gilsonian bon mot.