GSP are not maintained as a standalone module. But it has been forked and reintegrated in Grails.
GSP means GroovyServer Pages, which is similar to JSP (JavaServer Pages).
GSP Module Project has started originally by Troy Heninger.
The original sources of GSP module 1.1 can be found at Groovy's SVN repository.
There is also a new GSP project page: https://gsp.dev.java.net/