I just finished reading the book. Although it does appear, the linguistic subtlety you mention is lost in translation and I'd have found that particular passage confusing if I hadn't read your comment first. In short, at one point Glebsky figures he can definitively discover Brun's gender by asking him/her a question such that the answer would be something like "I had a nibble" or "I wolfed down two sandwiches". In the text, it's left unclear how one could infer gender from such answers.