Download the MIDI SysEx file for Korg X50 Split Sounds for use with the Korg GECIII. Follow instructions included with the file:
Links:
[1] http://www.soundtree.com/sites/default/files/GEC3-SysEx-X50.zip
[2] http://www.soundtree.com/sites/default/files/X50MacSysEx.sitx