Other than Tafsir Rasag, what else did Rav Saadiah Gaon write on Tanach? Did he write a full commentary on it? Can I find these writings online?
There is some question if Rav Saadya Gaon really authored some of the writings on Tanach attributed to him. The Chida in Shem HaGadolim writes that he has seen the commentary (mostly an Arabic translation) on all of Chumash written by Rav Saadya Gaon which is consistent with wherever he is quoted by the Even Ezra.
He also authenticates the existence of a commentary of RSG on Daniel and Ezra.
As far as the other commentaries on Nach attributed to Rav Saady Gaon, like many others, the Chida questions the possibility of Rav Saadya Gaon having authored them when they seemingly make references to Seforim that postdate him. In all his travels (where he came across many writing of earlier times that we don't have) he doesn't seem to have even seen any such commentary clearly authored by RSG himself.