The international casino game industry has adopted G2S (game to systems) protocol for the future communication standards between slot machines and gaming hosts. G2S protocol uses the internet protocols and XML message format. Not simply G2S protocol replaces the SAS protocol currently in use but with many limitations, but provides diverse application and business chances to the casino industry. In this paper, we introduce the overview of technical features of G2S system, and describe our design and implementation experience of core G2S protocol engine for slot machine devices. The G2S engine provides key protocol stack of G2S (HTTP, XML, and SOAP) and runtime services, using open source softwares and custom classes. The protocol compatibility of the implemented G2S engine is confirmed against an openG2S host software contributed by IGT. Also the measured memory size and perfroamcne is compact enough for typical slot machine hardware specification.