let’s assume he wrote it mid season 5.
gabriel might not be the brightest but i am pretty sure he KNEW nathalie didn't want to help him anymore (o he quite literally said in the finale "; knew you would betray me nathalie" so why would you... assume she would help your child
a letter to max and now a letter to sabrina?? idk guys would it be crazy to think that adrien’s letter is written by lila as well. she has all the information she needs and her whole has been impersonation