The move has nothing to do with current injuries, but instead is a way for head coach Norv Turner to prevent his best players from suffering setbacks that may prevent them from playing when the games actually count.
Turner cited the Chargers offensive line, which will be missing several starters, as the reason for giving backup Charlie Whitehurst the opportunity to take the opening snaps against the Vikings instead of Rivers.
As for Gates, he is currently healthy and Turner would rather not risk any injury.
NFL teams typically give their best players limited action in their final preseason tuneups, as the risk for injury is often considered higher than the potential gains from in-game experience. In Turner's time as Chargers head coach, he has not allowed Rivers to play in the last preseason game.