I don't see this cost in the case of Gersen. In the case of Ghyl Tarvoke and Gastel, yes, because their quests undermine their society, and thus there are some real losses to go with the gains. Ghyl at least can get out, but Gastel can't.
There are no losses with Gersen except the sense of purpose that has motivated him for several years. The fact that he is let down after accomplishing it is natural; a universal emotional reaction to the finishing of a big important task. His world hasn't changed, he's got a girlfriend, he's rich, and yes, he has to find a new occupation, but this is sort of an everyday thing under the circumstances.
As for Adam Reith, there is not even this. He gets exactly what he wants. If he tires of Earth, he'll just go back to Tschai. The reader, on the other hand, is let down; he wants to hear more!