like fastcgi, scgi is a protocol that connects web servers with other applications. compared to fastcgi its full specification is far simpler and it is easier to implement. compared to cgi, where each request runs application code from the beginning again, scgi allows for an application to initialize once and keep running, saving overhead
scgi works with a server in your application that processes scgi requests from the web server and sends scgi responses.
guiles (web server) seems http-centric and not suitable to build an scgi server.