It’s always been my understanding that the Res Gestae were the literal reminders of the the events and deeds he purports to have accomplished and then reflected in architectural allusions and symbols that he had created around Rome. The inscription itself is even part of the sundial construction of his mausoleum and ara pacis. Annual reminders for the people of his accomplishments. Personally I always interpreted it as his “index” for his version of The Gallic Wars which was the symbolism in the architecture and practices he left behind.