Do treadmill distance edits in GCM contribute to calibration?

Only on device i think

That's right, only on device contributes to calibration. Garmin uses 1 calibration factor so in theory is your next workout was identical (pace changes, distance, etc), Garmin will show the correct distance once you calibrate today's distance. Personally I think they should have a graded calibration that understands what your pace is at 180 spm vs 165spm

Garmin uses 1 calibration factor so in theory is your next workout was identical (pace changes, distance, etc),

So for interval training it will be never accurate? 

About GCM are you speaking about this calibration? 

You can try it next time while doing intervals. It might be close enough.

No, I meant that when you save a treadmill workout where your watch shows 1.00+ miles, and you choose to calibrate, Garmin scales the wrong distance using one factor. So it's life this: you run 6 miles but the watch shows 5 miles. You fix this using calibrate + save. So the factor Garmin saves is 1.2.  Or maybe 5/6 depending on how the factor is applied. So now your watch will use this factor for your treadmill runs until you recalibrate.

I assume you are talking about WDR (wrist dead reckoning - using the built in accelerometers rather than a footpod)?  Even a simple Garmin footpod will give much more accurate and consistent data on the treadmill than using WDR.  WDR also gets "calibrated" every time you run outside with good GPS coverage.  The calibration factor that you get from correcting the distance at the end of a run is on top of that.  The outdoor calibration bit is more complex, but the calibration factor from correcting distance I believe is just a single multiplier.  WDR has been getting better, but still far from perfect.  I wouldn't want to be basing strides off it.

So, also for run is possible to calibrate? I did not find this... 

No, nothing you can do for the outside run calibration.  This all happens automatically every time you run outdoors with good GPS signal.  But that's why they say you should do some outdoor runs before running on the treadmill.  I know this has had some problems and they've recently been working on it as noted in the updates.  It's never going to be as good as a cheap footpod though.