Following the instruction of an old mail from VL (Ellis),
I'm using now two GCP with 2 GCX.
CHAIN : GCP (old, 1.02 version) MIDI out to GCP (new) MIDI IN and MIDI OUT to GCX 1 - 2.
I'm pretty sure I know what's going on, but want to clarify something: by GCP "Old (1.02 version) do you mean an original DMC Ground Control?
In any event, looking at the MIDI signal chain as described above, your "Old GCP" is only configured to send outgoing MIDI information; there is no incoming MIDI connection, so nothing is going to happen on the "Old GCP" when using an IA button on the "New GCP." That's why I asked if the "Old GCP" was an original DMC unit, as they have no MIDI In jack, only a 7-pin MIDI Out.