Craig S. Keener translates ὅτι γέγραπται as "because the Bible says" in Galatians 3:13.
Craig S. Keener, Galatians: A Commentary (Grand Rapids, Mich.: Baker Academic, 2019), 253
Christ purchased our deliverance from the above-mentioned curse of the law, by becoming a curse on your behalf. This is because the Bible says, “Everyone who was hanged on tree is cursed.”