Replying to Technical outerwear info thread. (gore-tex)

I was just fucking around looking at jackets made by various companies.
The first thing I look at is the waterproofness.
So if a jacket is 10k waterproof it means what exactly?  10,000 mm of water on top of it won't permeate the membrane?  That's what I was told previously.
A lot of jackets just say they are gore-tex and have no waterproof rating.  It says gore-tex is guaranteed to keep you dry, so would it be safe to say it is at least 25k waterproof?
Cause some of the sessions stuff says it's 30k, but then it lists no rating and just says "guaranteed" cause it's gore-tex.
So I'm confused here cause usually if a product lists no rating I assume it's probably no good, but if it says it uses gore-tex I assume it's top of the line.

AKA: I have lots of time on my hands to do shit like this before college starts.