I have been investigated this a bit. If you look compare the fonts, there are a number of small differences: stem widths, cap height, ascenders, most obviously. The older Garamond Pro is taller, skinnier, and simpler (nearly rectangular stems irather than tapered). And many of the characters are a little bit wider.

If you blow them up (I found this on the net below and uploaded it to my profile), Garamond Premiere looks almost crude in comparison (These are at 300 DPI). But of course these are text fonts, not intended to be used at huge sizes.