The primary role of MGWSI is to provide the gateways used to integrate the Enterprise Web Developer (EWD) run-time environment and PHP or Java Server Pages. In this role, once you've installed and configured the MGWSI module and the appropriate gateway, you don't need to worry about how they are used by EWD: EWD and MGWSI should "just work".
You can, however, use the MGWSI product and its associated gateways for other purposes and you can take complete control of its in-built functionality. The MGWSI installation kit includes detailed documentation that describes its APIs if you wish to explore its functionality and capabilities. For example, Caché or GT.M programmers can use MGWSI and its gateways to integrate these databases with manually-written PHP, JSP or ASP.Net pages, or to integrate legacy MUMPS applications with Java or .Net.