Nope, no way to do that (Google Code can do that, AFAIK). The best you can do is:
a) make the gh-pages branch a branch of master (instead of resetting it) and just git rebase the gh-pages branch to the master branch every time you make an update to the master
or,
b) make the index.html page "smart" in the way that it is hosted on the gh-pages branch but is just an empty "shell" page which fetches its content from the master branch. I imagine this could be done via an iframe that covers the entire page and points to the index.html in the master branch (thus, effectively, the iframe becomes the page). Alternately, the webpage on the gh-pages branch could dynamically fetch content from the master branch via AJAX+GitHub API, and then inject the fetched content into itself (this is just for HTML content, since scripts and styles are easily injected via and tags).
I usually take approach a) for projects that are basically webapps.