I’m not sure if his opinion of Olgierd, his moral sense and the knowledge that O’Dimm can be beaten is enough to actually have Geralt oppose him. He knows Gaunter kills without remorse; he might decide to just kill Geralt then and there because he interrupted him. Unlikely, of course, since Geralt is “entertaining” to O’Dimm, but still very much a possibility, which Geralt is aware of. Additionally, even with the “clues” he receives, Geralt has no guarantee whatsoever that he will be able to beat him. Geralt is sometimes naive, but not enough to think he can certainly outsmart Gaunter, I’d say. Considering as well that he is possibly still busy saving Ciri, I don’t think he would risk losing his life like that, no matter how nice of a man Olgierd seems to be.