When gw tried to fetch that page the webserver returned a 404 Document not found to gw. Perhaps it was not there at the time gw asked for it. Find the time with
gw -st "select convert(id,'date'),Url,Reason from error where Url='YOUR_URL_HERE'"
and check your webserver's access and error logs for that time.
See the manual, http://www.thunderstone.com/site/gw25man/node100.htm
, to find how to determine what page caused gw to fetch the one in error.
The error table only gets cleared when you -wipe or create a new database that does not already exist.