The general concept is simple — Gig Performer passes through to plugins any program change that is not associated with a rackspace. When you have a single rackspace, by default its associated program change number is 0. That is why PC 0 is not reaching your plugins.
However, that’s not the whole story. Beyond program changes, there is a concept of bank selection, the goal being to allow more than the limited 128 choices you get when using a program change number.
If you change the bank number to something other than 0, then Gig Performer will not respond to the PC 0 because it’s not on the correct bank. Hence the PC 0 will be passed on to your plugins.
If you don’t understand banks, then a google search for MIDI bank select will probably help
