I don't know the answer to the thickness issue but I noticed the obvious visual difference of aftermarket vs GM/parts counter gaskets was the OEM stuff had a blue colored edging....kind of like what I remember from Fel-Pro "perma-torque" series gaskets...yea I'm that old
... I used aftermarket gaskets when I did my injector change out & I don't see where I'm having any sealing issues with the I/H's or the T/B or the plenum...yet...it's been about 3k miles of use since I did the work.
The one thing that struck me when I dis-assembled my top end(less cam covers & heads) was that almost every bolt was loose!?! It was as if no one used a t/Q wrench when it was assembled....as I have no experience with aluminium motors I figure that that is a bad trait...no resistance to thermal cycles between the alum. and the steel bolts or it was just sloppy workmanship.
Tom