What exactly do you mean by browser-based interfaces?
Access to the Gnutella-Network via a Browser needs a Program, which can be controlled via a http-interface (might be a good deal of work, but no more than for any other p2p-app).
If you mean: Implementing Gnutella in a browser:
Gnutella is a kind of search engine. You could implement a Gnutella search in a browser, by making it connect to the Gnutella-Network at startup. A search could simply result in a page, which lists the results. Downloads in Gnutella work via an only slightly modified get-request, and there are quite some free (and GPLled) Gnutella-Programs, some in Java, some in C:
http://phex.kouk.de - Java
http://limewire.org - Java
http://gnucleus.com/ - Windows, Microsoft Visual Studio 7.0 needed, COM based.
If you mean "Websites, which can be hosted in Gnutella":
Shareaza developed "collections", which are asically Websites, for which only a "*.collection" file gets downloaded, which is a zip including an xml-file, in which you can find magnet-links to all files which should be on that website. your client then downloads the files into the zip-file.
To distribute such a website, you need only to provide a source for the zip-archive with the xml-file, all other files can be downloaded via p2p (and even that initial zip could be served via p2p-networks).
Did it answer your question, or did you mean something else, which I didnt quite grasp?