gbs.php is a script to be used in conjunction with GaretJax's PHP Browscap.
It utilises the browser data that phpbrowscap parses from browscap.ini, to detect browsers on the YUI Graded Browser Support C-grade blacklist, and then stores this data in a cookie that can be queried to determine whether to withhold CSS and Javascript.
For more info see monospaced.com/labs/gbs.