Lonny Meinecke has written a tutorital for users of Harmony Assistant or Medlody Assistant and GPO:

Harmony/Melody Assistant

and GPO Studio Tutorial

I am a new user of Garritan Personal Orchestra, and would like to contribute a simple how-to for using GPO with a very affordable notation program called Harmony Assistant by Myriad Software. They actually offer two programs, Melody Assistant ($20) and Harmony Assistant ($70). Both are extremely powerful notation programs and both work with GPO. I was thinking such affordable programs might be of interest to folks (like me) who have a limited music budget but still want all the power and creative freedom notation programs offer when used with Garritan's software.
There are two ways to use GPO with Myriad's software. One, you can use live playback using a MIDI keyboard and use the mod wheel and sustain pedal as prescribed by the Garritan software to introduce live performance expression as you play.
The other, even more encompassing way is to use Melody/Harmony MIDI Parameter Curves function, which allows you to add expression and dynamics easily and precisely using simple notation.
To use Melody/Harmony and GPO:
1) Start GPO Studio (actually you can start GPO after Melody/Harmony too)
2) Load the desired instruments into one (or more) of the 8 slots of each of the 8 GPO player instance choices (you can change them later too)

3) Launch Melody/Harmony, go to the Configuration menu item, and choose Hardware Configuration to bring up that dialog
4) In each of the MIDI Output dropdowns, choose a GPO Studio instance (1-8), then click OK
5) Still in Melody/Harmony, click on the tiny keyboard icon next to the desired staff to bring up the Edit Related Instrument dialog
6) In the Output Device dropdown of this dialog (upper left) select the matching GPO player instance (MIDI 1-8)
7) In the MIDI Channel edit box of this same dialog (upper right), enter the slot number of the desired GPO instrument within the desired player instance
8) Enter some musical notation (can be done at any time)
9) Bump up the graphic Mod Wheel in GPO half to full so playback can be heard, and hit Play in Melody/Harmony. You should hear GPO instruments playing
10) To add nuance and expression, simply click the tiny green arrow icon next to the desired staff to bring up the MIDI options popup
11) Choose MIDI Specific A (or B/C/D/ or E - you can have 5 options)
12) Click the tiny green arrow icon again, and choose Setup to bring up the Edit Curves dialog
13) Make sure MIDI Specific A (or whichever you are using) is selected in the listbox
14) Change the Min/Max edit boxes to 0 and 127 to match the volume/expression range
15) Enter “Bn 01 ll” in the MIDI Data edit box (without the quotes) and click OK (this would be for the modulation wheel curve, for volume). This is a simple template for the actual MIDI command. The "n" will be filled in automatically with the channel number, and the two little L's will be filled in with the volume amount, 0-127, also automatically
16) Now you can draw your volume parameter curve. I know this sounds complicated, but really it's incredibly easy and intuitive. Basically, you just draw a little volume curve, and that's it. To add a control handle, click the staff in “Editing” (pencil) mode. To move a control handle, drag it in “Editing” (pencil) or “Select” (lasso) mode. To move a curve, drag its first control handle while holding down the Shift key.
17) After editing the curve as desired, play it back in Melody/Harmony to see how neat GPO sounds with note by note volume and expression.
18) To export the actual GPO audio, you have a bunch of choices. My favorite is to simply mute all staves except the one GPO staff you want to output. Then create a blank digital audio staff in Melody/Harmony, and record to that track as you playback the GPO staff in Melody/Harmony. You can now delete the GPO MIDI staff and adjust the new GPO digital audio track as desired. Do this for each GPO staff. Optionally, you can use GPO's record function to record the GPO portion of the audio to a file, and then mix it with your Melody/Harmony audio either within Melody/Harmony or in an external audio sequencing program. Even easier, you can use a utility like Total Recorder and capture the entire output during playback. I prefer the first method as it allows me to do everything in Melody/Harmony, and since the GPO tracks are each separate, I can individually adjust volume and pan as desired.
Note! You can FREELY mix GPO staves with Melody/Harmony soundbase staves, digital audio staves, what have you. This is really cool, since Melody/Harmony provides some really good GM soundbases. Also, you can add other MIDI expression commands using this same method too, just see the Garritan manual for the controller numbers and ranges.

You can access the tutial at Lonnie's website also and hear his music at: http://www.myriad-online.com/en/sharedxp/gpo/index.htm

Thanks Lonnie for writing this tutorial and sharing it with us!