Wikipedia
GBBS is a bulletin board system (BBS) program for the Apple II. Its first series, named GBBS, was written in Applesoft and used by boards such as Demon Roach Underground in Lubbock, Texas Its successor, GBBS Pro, was ACOS-based. GBBS-Pro was used by boards like ProBOARD II in Paso Robles, California, Scotland Yard GBBS/AE Pro in Cincinnati, Ohio and Apple Elite II in Riverside, California.
GBBS (literally: Greg's Bulletin Board System) was written by Greg Schaefer, who later authored the terminal emulation program ProTERM. The GBBS-Pro system was based on the ACOS compiler and language. ACOS was a BASIC-like language wherein the modem handling routines had replaced some of the other basic functions. Arrays (for instance) were unheard of in ACOS and so it was necessary to find other ways to work around these limitations (i.e. files replaced arrays). GBBS systems could be highly customized and modified. Mods were shared between sysops (system operators) and even ProTERM support was given via a GBBS system.