As noted, the Gore-tex jacket is not designed to provide any insulation. It keeps wind, water, and snow off of you, and the layers that you do wear for insulation, underneath. The ECWCS replaced several different items, so now the GI only has to carry the one ensemble instead of three. It's whole purpose is to stop wind and rain, and let perspiration out.
Prior to the Gore-tex, we had the field jacket and liner, the fish-tail parka and over pants, and the wool trousers and wool shirt worn with the rain suit. Each for a separate temperature and climate type.
Now with the ECWCS you have Goretex parka and pants, fleece shirt and pants, polypro long johns.
Not only is it a reduction in weight and bulk, but it's much more effective because it's easier to stay dry and adjust your clothing for the climate. With the "cold-wet" wollies and rain-suit, you were assured of being wet from sweating. That's why they made it wool, so it would still work when wet.
You can still use the old field jacket liner with the Goretex parka, just sew some buttons on it so you can button it up and use it as a layer. (ed to add: sew the buttons on the liner, NOT the parka. Buttons on one side, and use the existing holes on the other to fasten it on you, THEN put the parka on over it.)
If you're going to get one, buy a new one. Used ones in a surplus store are there for a reason. Usually leaks, or they're worn out. When you wear a ruck, or body armor, or whatever over it, the Goretex membrane will take a beating and delaminate. Once that happens, the parka will leak like mad. Usually it happens at the wear points, shoulder, back, etc.
Also buy the newer gen one if you can. The first gen had the big hood, and that fills with snow, or rain if you had it down doing stuff, or just wearing a helmet, headset, etc. The second gen has about as good a hood, and it rolls into the collar. The first one also had a lining that would start to migrate water from the sleeves and other openings. Yeah, it took along time for it to happen, but it happens and that's why the new one doesn't have that type of lining. You'll also notice the high-dollar goretex raingear for backpackers has a net lining for the same reason.
They're great if you use them right. If you don't know what you're doing, then you'll think they suck.
Ross