Possibly: check the MIDI channel to which the second instrument is assigned (the number in the drop down list on the toolbar). Then make sure your keyboard or other input device is set to the same channel.
The layout window shows midi channel one (1) assigned to the original instrument and channel two (2) assigned to the second instrument in the same GIG file.
When opening the instrument editor only the channel assigned to the original instrument shows in the midi window.
All midis from controllers are OK.
I wonder if there might be a bug here. When you try to edit the second instrument you get the visual feedback - you see the paremeters that change in the \"velocity map\" for instance, and you can select the different instruments by clicking on them, showing their individual parameters. All you will hear though is the original instrument.
Try this: with the GIG file loaded in in the instrument editor, switch back to GigaSampler (click on the task bar button), right click on the right hand end of the instrument/midi channel and select the second instrument from the menu which appears.
The alternative way is to send the appropriate program change message from your midi input device (probably program number 2).