The standard attack in this case is that someone could steal your cookies (if they are Js cookies) or execute instructions on gkoberger.github.io without your permission, since that is the context.

The way you would do this is create an answer to one of these questions on stack overflow that includes malicious javascript.

However, it's kind of unlikely there is much you can do with it. Sometimes a site will share cookies with subdomains, but this is not likely for github because you are allowed to publish arbitrary js there, so that would be a huge security hole.