Mark Walson, developer of MidiNotate, has written a thorough 35-page tutorial on "Using Garritan Personal Orchestra in MidiNotate Composer". This tutorial describes the procedure for configuring MidiNotate Composer to work with GPO, and then describes Composer features you can use to create MIDI files and scores that use GPO instruments. The tutorial is well illustrated with plenty of examples.
MidiNotate Composer is an intuitive, easy-to-use and affordable notation program and has had graphing capabilities for controllers for some time. GPO is heavily reliant on controllers for fine-tuning the performance of the instruments, and MIDI Notate makes that pretty easy with the graphing capabilities, the midi event list, and the special instrument key-switching and mid-track voice change capabilities that it has.