Goretex is a "Waterproof" fabric. There are many WP/B membranes, and they're rated to withold water to a certain pressure, measured in mm of H2O. A typical number for Goretex or Toray Entrant is 10,000mm. This just happens to be 10 meters, or (drum roll please) about 1 atmosphere of pressure.
Goretex, and indeed any WP/B membrane, tend to leak where you apply pressure for exactly this reason. Wear some Goretex pants and sit in a canoe in a downpour for a couple hours and your butt is going to be soaking wet, I promise you. Your shoulders will be, too, with the rain pounding on them.
A Goretex drysuit will not stay all that dry when used for diving and there's a constant, significant pressure gradient across the fabric.
BTW, as someone else mentioned, all WP/B fabrics are made from a few different layers. You start with a nylon or polyester, then laminate the WP/B membrane to the inside of the fabric. This is the waterproof part. The exterior of the fabric is covered with a DWR, which is essentially Scotch Guard. This helps the water bead up and roll off the nylon. The nylon can get soaked without the waterproof membrane allowing water in, but the breathability of the fabric will obviously be comprimised by having a soaking wet nylon on the outside of the breathable membrane.
