At best you want an XML editor that is able to validate the XML for errors against an XML schema. Unlike GalCiv 2, for GalCiv 3 Stardock provides XML schema's, which allow you to see what changes are possible within the XML file. GalCiv 3 will crash if an XML is not valid, describing what does not validate with the XML schema.

Most suggest Notepad++: https://notepad-plus-plus.org/
ed4053: There is an xml plugin for it that includes syntax check and validation (both can be done automatically too) here: http://sourceforge.net/projects/npp-plugins/files/XML%20Tools/

Though currently I am using Sublime Text 3. The text editor provides enough maneuverability around any type of project. Such as switching to the XML schema by copying the name from the second line of an XML file, pressing Ctrl+P, pasting it and picking from available options.
[edit] The following package can be used to have validation of both XML and XML against the schema that has a relative location, you will also need SublimeLinter and xmllint:
https://github.com/jtojnar/SublimeLinter-xmllint 

I had also tried Altova's XMLSpy XML editor, which you can trial for 30 days, It is meant for use in business as you can tell from the price. It has some nice views and you find out that Stardock's provided schema files are not complete enough.