Git commits include name and email address of the author of the patch as well as the person committing the patch. Unless you set up your git client with your email address, your commits will have useless email addresses like "firstname.lastname@example.org".
The exact error message you get if you try to push commits without useful email addresses to GNOME repositories is:
The commits you are trying to push contain the author email address '$email'. Please configure your username and email address. See: http://live.gnome.org/Git/Help/AuthorEmail For instructions about how to do this and how to fix your existing commits.
To ensure that the history has proper emails, all people making commits should run
git config --global user.name "Your Name Comes Here" git config --global user.email email@example.com
on each machine they use git on.
By default git uses your configured user name and email address both as commit and author information. If you are committing a patch written by someone else, you can correctly attribute authorship by using the --author argument of "git commit":
git commit --author 'Author Name <firstname.lastname@example.org>'
Fixing existing commits
If you have existing (unpushed) commits, which you need to fix, you have several options.
In the case of just needing to fix the last commit
git commit --amend --author 'John Doe <email@example.com>'
If there are several commits which you need to fix, options include: use "git rebase --interactive", mark the commits for editing, and amend each one. Or use "git format-patch", edit the patches, and use "git am".