From p8-p12 of The Tunnel, Gass quoted a series of diaries from writers in history (William Wordsworth, Katherine Mansfield, Alice James, Virginia Woolf, etc.) and wrote his corresponding comments. How should we understand the selection of the diaries and the comments?
It is very important to remember that this entire book is written in the stream of consciousness technique. In this way, we are let into a character's thoughts. We cannot, and will not, know why the character thinks the way he does. It is simply fun to take the ride along with Professor Kohler. In this case, Professor Kohler is talking about diaries that he is familiar with in his field at the college. Let's look at the diary entries and figure out a little bit more about them.
In truth, the details about the diaries and even who wrote them are unimportant. What is important to remember, however, is that this is the perfect example of stream of consciousness technique. We see an author's diary entry, and then the characters comment in his mind on that diary entry. In this way, professor Kohler talks about everything from death to chairs, to the Bible, to Jesus, to many other things. There are many interjections given along the way (such as "what the hell"). It is simply the way his mind works as he reads these diaries. Again, it is a perfect example of the author's ability to write stream of consciousness.
In conclusion, remember that these are only my ideas about the characters' thoughts and how we should decide to interpret them, as you ask in your question. We can never be fully sure about things, because this entire book is written in the stream of consciousness technique.