is it possible to add a new local branch to track the remote branch created by github pages?

You can create a branch by using git branch:

$ git branch gh-pages

And then check out to it:

$ git checkout gh-pages

or use the -b flag to perform both operations together:

$ git checkout -b gh-pages

Once you’ve done that, you can perform changes and commit them, and then use the branch name when pushing:

$ git push origin gh-pages