What do you think Gilbert's function is in The Thin Man?
There is a grand clue in eNotes character sketch of Gilbert Wynant (often referred to as "Gil") as found in The Thin Man:
Gilbert Wynant, Mimi’s son, an odd, extremely inquisitive young man.
Hammett has an interesting way of using his narrator (who is often boring as heck). The narrators of Hammett's novels are often devoid of opinions, often acting as deadpans or, as I like to call them, "wet mops." Because they have no opinions of their own, this gives the reader ample opportunity to learn more about characters through the narrators interaction with those characters, most especially through dialogue.
The Thin Man is no exception: Nick Charles in another one of these narrators (even though he is sometimes humorous). All Nick does is report facts. Nick NEVER talks about how he feels about those facts unless he is speaking with another character. This is where Gilbert Wynant comes in. Because Gilbert is both "odd" and "inquisitive," he tends to ask Nick the most probing of questions.
Gil: Could I come down and see the body? I've never seen a dead body. ... Well, I've been studying psychopathic criminology and I have a theory. Perhaps this was the work of a sadist or a paranoiac. If I saw it I might be able to tell.
This quirkiness and odd intelligence of Gilbert has two effects: 1. It creates suspense for the reader due to Gilbert's odd nature. 2. It becomes an interesting method for the reader to gain information, not through the narrator, but through this eccentric character.
Thus, if it weren't for characters such as Gilbert Wynant, Hammett's works of literature would be a lot less exciting, less informative, and even more confusing.