Unfortunately most of these posts no longer apply, because Gedit is now even better than before, as Gutsy Gibbon is now shipping with a new and improved version based on the GTKSourceView2 widget, which allows for context sensitive syntax highlighting, and big speed improvements, amongst other things.
The first thing you'll want to do is use your favorite package management front-end and install the gedit-plugins package, which includes a few extra plugins. Then go here, and follow his instructions on installing the mime types and lang files. He also has a few snippets, and other tips which are quite up to date. If you are running rails 2.0 you'll also want to edit his mime types file and add a couple of glob entries to handle the new view naming scheme.
<comment>Ruby XML Template</comment>
You can find many more snippets here, and in case you don't understand Spanish, here are the files. You will only want to use his snippets and discard the rest though. Place them in '.gnome2/gedit/snippets'. There are other snippets around the web if you just google a bit. You'll probably end up customizing them a lot, i did. Also be sure to download the textmate-like syntax highlighting style. Place all downloaded styles in '/usr/share/gtksourceview-2.0/styles'.
Now just open up 'Applications->Accessories->Text Editor' and then in Gedit, 'Edit->Preferences' and check all options under View.
Then under Editor set your tab prefs, I like a width of 2 and I also check insert spaces instead of tabs, and enable automatic indentation.
Under 'Fonts and Colors' you might want to enable Darkmate or Oblivion. You can also download the monaco font used on apple, but remember that much of what he says in the post no longer applies, so tread carefuly.
Then under plugins be sure to activate: Bracket Completion, Embedded Terminal, External Tools, File Browser Pane, and Snippets. There is a Gemini plugin out there which does kind of what Bracket Completion does but I prefer the latter. You might also want to check out Session Saver, Class Browser, and Snap Open plugins. Check out this list of Gedit plugins.
You should now be good to go! A big heads up to Joselo, and Robert from Nationcode and Insane Terminology respectively, and Luigi for the textmate theme!