GSE is Google Servlet Engine which according to Google is “their own fast, lightweight servlet engine”

Sources

http://google-opensource.blogspot.com/2009/01/opengse-released.html

http://websystemsengineering.blogspot.com/2011/03/http-headers-server.html

First encountered in the HTTP server signature field under the Miscellaneous heading from a SSL Labs SSL Report for mail.google.com.