I my post on twitter I was incorrect, so just decided to post it here:
Gedit indent - select lines & just press TAB
Gedit unindent - select lines & just press Shift+TAB
If you are using Gedit for coding, you can find my post about Gedit templates plugin useful.
I am not sure how many people finding Gedit good enough as an editor for programming purposes, but I am using it exactly that way and quite happy with its functionality. Especially I like its integration with gnome virtual file system, that allows it to work directly on remote hosts, and ability to use and create new plugins for it.
Here is a brief description & download link of template plugin I have written for myself. So, you may find it useful too.