Simple clone the wiki of that project:
git clone https://github.com/github/gollum.wiki.git
You will get .md
files there.
C:\Users\VonC\prog\git\gollum.wiki>dir
29/01/2014 07:44 <REP> .
29/01/2014 07:44 <REP> ..
29/01/2014 07:44 0 .md
29/01/2014 07:44 2 hello.md
29/01/2014 07:44 11 649 Home.md