There used to be a midi translator file for GPO. Some information is below
This is a java applet so you'll need to download .
I do have a copy so you can email me privately if you want me to send it to you. I never used it so cannot pass judgement on it
GPO Standard MIDI File Translator
The Garritan Personal Orchestra MIDI File Translator is a utility that you can use to translate any standard MIDI file to a new file that is optimized for playback with GPO. Standard MIDI files use note-on velocities (how hard you press a key on a keyboard) to determine volume at any point in a track, whereas GPO uses Mod Wheel to determine the volume. The MIDI File Translator translates the note-on velocities into Mod Wheel events that GPO can understand. After you translate a standard MIDI file into a GPO optimized file, your playback of that midi sequence with GPO will be improved immensely!
Using the MIDI File Translator
On Windows, you must download and install Java from www.java.com, before you will be able to start the translator.
usage paragraphs deleted here to save space but included in the full file
Limits of Standard MIDI Files
Standard MIDI files (unless created using GPO) are not necessarily the best match for Garritan Personal Orchestra if you wish to hear the very best music that is possible with GPO.
As always, you will be able to obtain the best results from GPO if you create your own MIDI sequences by recording the parts yourself, rather than using a pre-created MIDI file. Any MIDI file not created specifically for GPO will be missing the important and fundamental features to GPO that you can add when creating your own sequences, such as legato, crescendos/decrescendos, and attack speed variations, just to name a few. Please read the Garritan Personal Orchestra manual to understand the full controls that are available to you when playing and recording parts. Using and understanding these controls will make an amazing difference in the quality of sound you will achieve.