your rewrite rule needed to be edited to include the [QSA] flag:
RewriteRule (.*)/ index.php?GSG_URL=$1 [QSA]
When the replacement URI contains a query string, the default behavior of RewriteRule is to discard the existing query string, and replace it with the newly generated one. Using the [QSA] flag causes the query strings to be combined.Consider the following rule:
RewriteRule /pages/(.+) /page.php?page=$1 [QSA]
With the [QSA] flag, a request for /pages/123?one=two will be mapped to /page.php?page=123&one=two. Without the [QSA] flag, that same request will be mapped to /page.php?page=123 - that is, the existing query string will be discarded.
http://httpd.apache.org/docs/current/rewrite/flags.html#flag_qsa
