Gedit XML Tools is a plugin for GNOME's text editor, Gedit, which is going to include many useful tools for xml development. XML validation (DTD, XSD), Xpath, XQuery etc. The plugin (version 0.1) is currently able to do XML validation (XSD only) and do evaluations of Xpath queries.
At the time of writing, the initial authors have no motivation and/or time to extend this plugin. If you are interested in the development of this package, send us patches and we will give you commit access.