In the 1980s, Games Computers Play (GCP) was an online service and one of the first multiplayer online games (MOGs) to offer a graphical user interface (GUI).
The system was primarily accessible with Atari 8-bit computers with a minimum of 48k of memory. A version for the Atari ST was also available late in the service's life. The service only garnered about 1,000 subscribers at its peak.
References
Games Computers Play Wikipedia(Text) CC BY-SA