I know some other text editors like Notepad++ enable split pane to edit to files side by side. Is there any way I can add this functionality to Gedit?
In gedit, choose Documents > New Tab group or Ctrl+Alt + N your gedit will split vertically. And you can drag tabs.
Update: Gedit has now added this feature. I'm keeping my answer around for those using older versions of the application.
Gedit does not come with this feature.
But there is a plugin you can get: http://live.gnome.org/Gedit/Plugins#line-505
Not yet, but it's in development see here, someone's working on it (among other features) as a GSOC project as far as I know.