Well it's not exactly what you want with gtk, but there was an attempt to do this with Perl/Tk called the perlplus plugin for Netscape and Mozilla. Get it and test it perlplus

Now it has alot of problems, especially with security, which make it useless for general purpose use, because you need to specify a security-cgi location specific to each plug-in. Which means that everyone would have to have a plug-in for EACH different web-site. However, if you read about how it is done, there is a general purpose method for creating plug-ins for Mozilla, and maybe that is what you might want to look into.

But in general, I think it is better to just design a custom app, which the end-users can download and run, connecting to your site through sockets, or your web-server, giving you a GUI connection.