GSP are not maintained as a standalone module. But it has been forked and reintegrated in Grails.
GSP(GroovyServer Pages) is similar to JSP(JavaServer Pages).
GSP Module Project has started originally by Troy Heninger.
Troy's GroovyPages project page is http://www.investortech.com/emp/troy/groovypages.htm
The original sources of GSP module 1.1 can be found at Groovy CVS: http://cvs.groovy.codehaus.org/viewrep/groovy/modules/pages/src/main/groovy/modules/pages
Now here is a new GSP progect page: https://gsp.dev.java.net/