I want to sync a repo I forked on Github with the original.
Is there any way I can do that using only the Github for Mac client and/or the Web interface?
If not, is there a GUI client (for Mac OS X) that I can use to do that?
GitHub for Mac does not support multiple Git remotes. It only works with the origin remote.
I read through the Git OS X clients listed in this answer at SO and culled the ones that indicate support for remotes and push/pull.
Gitbox push and pull, rebase, branch reset and cherry picking
obviously this is quite easy With the commandline:
I've personally used SourceTree and highly recommend it. It's packed full of features, and is compatible with multiple repo services including kiln and github. The program is very easy to use, and highly supported. They even have their own Q/A site where if you run into problems you can ask questions (the interface is very similar to Stack Exchanges very own).
I use kiln solely, and have never dabbled with github, so I'm not for sure what it is exactly that you're trying to do but I do know that you can fork, and merge forks in the repo using the Git and that SourceTree supports doing that as well. I would look into that functionality and see if that is what you are looking to do.
Might I suggest Bazaar?
With the git plugin, the Bazaar GUI (or CLI) will work quite well with Git. I have used it to fork a Git, but not to sync back with it (although I know it supports this behavior).
You could build a Bazaar branch from you local Git fork, and then use bazaar to 'push' your new branch back to the original.
I am not 100% sure if the Git plugin is included with the Mac install (since I dont have a Mac). But I know it is for Windows, and I have installed the plugin to Linux painlessly.