You can use the following system exclusive messages to turn off or on parts in Roland GS sound sources. You will need to input and send these from a sequencer that allows you to create SysEx messages from scratch.
To Turn a Part Off
Part / Message
1 / F0 41 10 46 12 00 00 18 15 00 53 F7
2 / F0 41 10 46 12 00 00 19 15 00 52 F7
3 / F0 41 10 46 12 00 00 1A 15 00 51 F7
4 / F0 41 10 46 12 00 00 1B 15 00 50 F7
5 / F0 41 10 46 12 00 00 1C 15 00 4F F7
6 / F0 41 10 46 12 00 00 1D 15 00 4E F7
7 / F0 41 10 46 12 00 00 1E 15 00 4D F7
8 / F0 41 10 46 12 00 00 1F 15 00 4C F7
To Turn a Part On
Part / Message
1 / F0 41 10 46 12 00 00 18 15 01 52 F7
2 / F0 41 10 46 12 00 00 19 15 01 51 F7
3 / F0 41 10 46 12 00 00 1A 15 01 50 F7
4 / F0 41 10 46 12 00 00 1B 15 01 4F F7
5 / F0 41 10 46 12 00 00 1C 15 01 4E F7
6 / F0 41 10 46 12 00 00 1D 15 01 4D F7
7 / F0 41 10 46 12 00 00 1E 15 01 4C F7
8 / F0 41 10 46 12 00 00 1F 15 01 4B F7
Owner's Manual: https://www.roland.com/us/support/manual_archive/