But old Gmail did override this.
There are good design reasons for this. The goog.editor package is designed to be modular. This tab-to-indent behavior is instead provided by the goog.editor.plugins.SpacesTabHandler plugin.
As I mention before, I find it questionable that the GMail developers elected to omit this plugin. I'm not suggesting that there are some fools on the project who can't read an API. Rather, it strongly indicates that GMail is catering less and less to any approximation of the "power user", and assuming that most emails could just as well be composed in a <textarea>.
[1]: http://ckeditor.com/demo [2]: http://www.tinymce.com/tryit/basic.php [3]: http://try.sencha.com/extjs/4.1.1/docs/Ext.form.field.HtmlEd...
I agree that it would be nice to have some way to force insert a tab, but it's hardly surprising that it works the way that it does.