Greetings Stamps were a Royal Mail marketing experiment to encourage social mail and meet the needs of big retailers such as WH Smiths, which wanted to sell more interesting stamps. The idea was to offer the public attractive stamps, available all year round, to use for letters and greetings cards. Greetings Stamps were first issued in 1989 in books of ten stamps. There were discontinued in 1997.
This is short for General Post Office, which was what Royal Mail was previously called.