I have been confused about why the principle of sufficient reason plays the role it does in Garrigou-Lagrange's work, especially on why and how he reduces it to the principles of contradiction and then identity.  I was speaking with Fr. Lawrence Dewan at the P.A.S.T.A. Conference in Houston and he mentioned to me that Gilson's students had some sort of record or notes about the similarities between G-L and Afrikan Spir's work on Leibniz.  I haven't found Spir's work on Leibniz, but you can find interesting connections with G-L's discussion of sufficient reason in Spir's Pensee et realite:  https://archive.org/details/penseetralitess00spirgoog .  Compare this not just with G-L's God His Existence and Nature, vol. 1, but also https://archive.org/details/lesenscommunlaph00garr.  Incidentally, Spir was a great influence on Nietzsche as well!