The Loeb edition bears the inside title (after the 50pp of introductory material) "Ammiani Marcellini Rerum Gestarum libri qui supersunt". (The volume titles are merely "Ammianus Marcellinus", although they include no other work by him, since none has survived.) This inside title translates as "The books that survive of the Deeds/Acts/History of Ammian Marcellinus". Nowhere have I been able to find where this Latin "title" comes from; I can't remember reading or hearing anyone actually refer to AM's History as "Res Gestae", although that's a Latin phrase that can be translated history.

The "Divi Augustae" (sic), on the other hand, is a much clearer mistake; which should have been — so to speak — "Divi Augusti", although FWIW (not very much) one can find "RG Divi Augustae" online. Still, it's not right, and anyhow RGDA always refers to the piece of Augustan propaganda.

I"ll revert the Loeb "Reference"; although my edition doesn't have the same ISBN numbers — prolly better to leave those off.