The GBBS Pro Source Code archive has moved. The new location is on the Call-A.P.P.L.E. Github page which is located at:
GBBS Pro Bulletin Board System is an Apple II based BBS program which allows users to run their own bulletin board cheaply and easily.
The latest version of the publicly available builds from the GBBS Pro website at https://gbbs.applearchives.com/ or order the manual for version 2.2 from A.P.P.L.E. at https://www.callapple.org/documentation/books/gbbs-pro-2-2/