Gnutella v0.6 said a query fo 4 blank " " means asking for a list all shared files. Recently, I noticed there exists a great amount of query string "\" in Gnutella network. What does this query mean? I don't think any person would send such a query to ask for files.

They are sent by BearShare clients. Some Gnutella clients, especially those that do not support GGEP extensions, will drop empty queries. If one is looking for a file based on the hash ( located in a GGEP extension) and not a keyword, the query might be dropped (since there's no keyword specified). So instead, the query contains at least one character (the "\") which prevents the query from being dropped. A "\" could therefore be considered a "blank" query.