QML files opened with gedit don't have syntax highlighting. Is there a package providing a gtksourceview lang.spec file for QML? If not, has anyone written a lang.spec than can be installed locally?
I know this doesn't directly answer your question, but there are several other editors that do support QML syntax.
The best option in my opinion is to use Qt Creator's (the Ubuntu SDK application) editor component. In addition to QML syntax highlighting, it also as context-sensitive completion.
On the other hand, if all you want is a simple text editor that offers QML syntax highlighting, KDE's Kate is a good solution that I use. While it doesn't integrate perfectly with Unity, it still is a nice editor.