Sheet Music software
 

Does anyone know about a GTK based sheet music editor?

Have you checked freshmeat.net or maybe sourceforge? I never really thought about it before, but that's something I wouldn't mind having around from time to time myself.

Check out GNU LilyPond and its GTK+ frontend Denemo.

Does my brother have to take his hands off the keyboard?
He currently uses Noteworthy Composer in Windows and
I want something with similer look and feel.
The feature in Noteworthy he uses the most
is selecting an entire set of notes and moving all of them up or down
so that transposition is easier.

There is also rosegarden
It looks like it is more than just a sheet music editor.

rosegarden is QT/KDE. I use GTK/GNOME exclusively.