A basic googling didn't yield many good results. If you are in a typical workspace, and open a terminal tab you can easily navigate back one directory and find the .gitconfig file.
The problem is, anything you do in there doesn't seem to persist to other workspaces; it only works for that local workspace.
Is there a way to set up configurations for Git globally?
Each Cloud9 workspace is like a completely new Ubuntu machine and is self-contained. Any changes you make in one would not affect your other workspaces. The changes made to git would be global within that workspace, and will impact any and all git repositories created within, but not repositories from other workspaces.
It's like any changes you make to one of your laptops will not impact your other laptops, similar is the case for Cloud9 workspaces.