You can't do a client side detection in the browser as to whether or not they have it installed on that page. The way I would do this, is to have a download page which is separate, internally accessible and then you could add a HIP check (if you have that license) which says if you already have the client, put up a block page. Again, the links to the GP clients on the portal page are all you get, so having an internal page, maybe even one with LDAP auth is pretty easy to make, then on the hosted server you could allow/deny rules with the HIP check or have multi-language or whatever you want. Make sense?
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