I would go with T-Gonz as your flex. Bush has a tough matchup and doesnt see many touches. Moreno is coming off an injury and may not play, definitely wont be 100% even if he does. As for Alexander, he has a tough matchup as well and is fairly low on the targets pecking order in StL.
I like Gonzo also but if you like to gamble...Alexander is looking like he is going to get a boost in production...which might mean more receptions and possibility of a touch. I am not so sure Gonzo is going to have the monster game he had last week.