Oh look, I'm tagged...... Now to answer your questions,
A canned tune is a pre set tune. Every 3800 performance company has their own version. They all have some lean areas enriched, trans shifts adjusted and firmed up, a thousand little tweaks, not specific to your car, but designed to work with any wbody at x level.
As far as pcms. If you use your gt injectors you can drive the car with your gt pcm, just leave off the supercharger belt until you get it tuned . If you want to use the gtp pcm you will need to use the gtp map sensor and wiring, gtp injectors, boost bypass valve and wiring, and somehow link that PCM to your BCM(sometimes the car will run without linking them sometimes it wont). You would still then need to go get a tune before any real aggressive driving (once again, you could be fine, but if not you are risking a blown motor).
Hope that helps
P. S. Find someone on the forums to tune you, will be much better than a zzp tune. [MENTION=109]TommyGloves[/MENTION] and [MENTION=517]10sec[/MENTION] used to do that as a side business.