There's a new Microsoft Knowledgebase article that talks about how to use the unsupported gthrlog.vbs tool. From the article:
Every time that SharePoint Portal Server creates an index or updates an index, SharePoint Portal Server creates a gatherer log entry in a table in the component settings database for the portal site. Log entries contain data about URLs that are accessed while the index is being created. The log can record successful accesses, indexing warnings, access errors, and accesses that are disallowed by rules.
By default, SharePoint Portal Server 2003 logs index warnings and access errors. If you want to log successful accesses or accesses that are disallowed by rules, you must configure these options.
You can view the content of gatherer logs on the Gatherer Log Details page of the portal site. You can also use the Gthrlog.vbs tool to view the information that is contained in the gatherer logs. Gthrlog.vbs is an unsupported tool that is located in the Support\Tools folder of the SharePoint Portal Server 2003 CD-ROM. Gthrlog.vbs is a Microsoft Visual Basic script file that connects to a database that is stored in Microsoft SQL Server and retrieves the logging information from the tables in the database.