Greylisting is a way of reducing spam coming into users' mailboxes on the mail server. gps (it soon will be called greylist) is a fast and secure implementation of a greylist policy service for the postfix mail server.
gps is no longer actively developed. Use the Greylist daemon instead. The greylist daemon is completely compatible to gps. The same database schema is used and any configuration that was possible with gps can be done with the greylist daemon (but is slightly more complicated).