Pack fitting is alot like shoe fitting. Manufacturers differ greatly in their fit philosophy. Just like shoes, it really is determined by the manufacturer (the same can also be said about the method manufacturer's obtain pack volumes). As a side note, this is a huge reason why it makes sense to spend up on a higher end pack because most of these manufacturers put a tremendous amount of resources and effort into fit. A lesser quality pack is far less concerned with this. I could give you many examples.

Since I've never owned a Gossamer Gear pack, I don't know how they fit. The best way to get an idea is to ask someone who owns one and understands it's fit based on their size. There really isn't a standard measurement industry wide that determines a pack's size. I would never buy a pack based on a size measurement without trying it on first, unless you got an incredible deal and it can be returned without issue.