It's been several days and still have not received an explanation about what you meant by this
It was just one day from your post that said you did not understood what I meant, to the one that said "It's been several days"...
Since you mentioned that assigning this key by another addon will keep the Z key from working in GSX, which I assume is what you meant, I went back and tried to see if I could find where I may have assigned the Z key.
Well, not exactly. I think you didn't understand how this could happen, because you don't recall having personally assigned any key to another addon but, that's not what I said.
I've said "assigned to something else
by another addon", which means any other addon can try to reserve a key for itself, without any intervention on your part. What usually happens, the addon loaded first will get the key, and the others that require the same key won't respond to the same key.
The plane I was using at the time to set the parking position was the PMDG 737NGX. So, I thought I would switch to another aircraft since this problem came up when using the NGX. After switching aircrafts, the Z key worked in GSX. So it had to have something to do with the PMDG 737NGX
I was quite sure the only possible reason for the Z key not working, with the rest of GSX working, could only be caused by another addon taking the key for itself. And in fact, this is what happened.
In the FMC, I found that the Z key was assigned to the auto-pilot disconnect switch, which I figured was the default in FSX anyway.
That's the default FSX key, yes. But by default, it's supposed to be used ONLY in regular flight mode, not Slew mode. Slew mode has a different set of key assignments so, by default, the Z key it's assigned to lower the plane altitude slowly.
The GSX editor works by setting the airplane is Slew mode, so it uses the Slew keys, not the regular flight keys. Q/Z raise/lower the plane altitude so, it was only logical we used them to raise/lower the *object* altitude.
But it was still hard for me to understand why assigning this key in the FMC would have any effect in GSX
It shouldn't but, from your report, it seems the PMDG takes over the Z key even when in Slew mode, not just in regular flight mode, like the default auto-pilot disconnect function does.