Trading Gibson will be a clear step back in goal. The goalie in return in any trade will obviously be worse than Gibson and his replacement next season, whether it's this goalie or another, will almost certainly be worse than Gibson.
It stands to reason that unless Dostal compensates by playing more games, then the Ducks will allow more goals after a Gibson trade. Considering their current reliance on stellar goaltending and inability to consistently score goals, it seems like trading Gibson would be a mistake for the team. However, if he wants out, that should be respected.
Consequently, I understand why Verbeek will not lower his value on him. Trading Gibson hurts the Ducks now and next year. It should cost a lot if the Ducks are having to take a step back in goal.