I'm not 100% positive on the specifics, but I've never seen it used. I suspect its not possible.
(context: I make https://circleci.com - a continuous integration company for web apps, often Rails. We occasionally get support requests that allow/ask us to look at Gemfiles, so I've seen an above average number of Gemfiles. However, I more often see the stdout of the `bundle install` command, which shows GitHub being accessed).
You can also specify multiple gem sources (http://gembundler.com/v1.2/gemfile.html), but usually only rubygems.org is used unless you need a private geù repository.
Yeah the question is, can you specify to grab rails from github with a fallback if it is not up? So the same system it seems to be using with multiple gem sources but on a gem by gem basis like you would do with a github repo.