I have been reading Gilson's book, "Being and Some Philosophers." Throughout, the work he seems to be holding up the distinction between essentia and esse as the solution to all our metaphysical difficulties. Leaving aside any disputes one might have about his understanding and application of the principle, it seems that he has neglected a somewhat fundamental question: Why do we think that this distinction is a true distinction? He admits, and I think rightly, that this distinction is a judgment. If it is a judgment, then it seems that we ought to be able to have confidence that it is true. Now, it does seem that it is among the first things, and in that sense it could be seen as self-evident, but it seems that it should be considered as something self-evident to wise, and that we should have some way of manifesting a need for making such a distinction. It seems, moreover, that such a an argument would a reduxio.

I therefore pose three preliminary questions: Does it seem right that we need to make an argument for this distinction? Is it correct that such an argument must be a reduxio? Is there anyplace where someone makes an argument manifesting this distinction?

Assuming an affirmative to the first two questions and being ignorant of the third, I formulated the following argument, which I give in brief outline:
1. I take St. Thomas Third Way
2. From this we see that there is a necessary ens per se.
3. Such an ens cannot not exist.
4. What it is to be this ens is to be.
5. Therefore there must be a distinction between the essentia and esse of all other entia.
6. If not, then they would be necessary through themselves.
7. But this is absurd since they are possible and necessary through another.
8. Therefore, there must be a real distinction between essentia and esse in all caused entia.

I think it might be the case that the movement from 3 to 4 will require that the simplicity and perhaps unity of God be shown. The simplicity would not be understood fully however, until we were sure of the distinction. Rather the argument for God's simplicity would be something like the second argument that Thomas gives in Q.3 a7, Utrum Deus sit omni simplex. The argument is as follows: Secundo, quia omne compositum est posterius suis componentibus, et dependens ex eis. Deus autmen est primum ens ut supra ostensum est. As I look more closely the third fourth and fifth arguments seem like they could all be made without making the distinction between esse and essentia.
However, once the distinction between essence and esse was made, then we could go back and manifest more clearly what it means for God to be wholly simple.

To clarify: I think this argument gives greater certainty to the distinction between essentia and esse. I think that we can probably see this distinction in a vague way before hand and thus we would be looking for an argument to manifest it more clearly and certainly.
One text that might challenge the argument that I am giving is Boethius' De Hebdomadibus and Aquinas Commentary on it.
Here is a decent translation of Boethius' work (I could not find the text in Latin).: http://www.fordham.edu/gsas/phil/klima/medphil/Boethius.htm
And here is St. Thomas' commentary on the same.

Boethius seems to imply that the reason he gives for the distinction is enough, and St. Thomas certainly does not correct him.
Boethius says, "Being and that which is are different. For being itself does not exist yet, but that which is exists and is established when it has taken on the form of being. "

I eagerly look forward to you illuminating insights!
Pax Christi Vobiscum