your rewrite rule needed to be edited to include the [QSA] flag:
RewriteRule (.*)/ index.php?GSG_URL=$1 [QSA]
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.