I have never owned any Gore Cycling products. I am 5'9'' 175lbs with a muscular build and I bought an XL. I wear an XL jersey in Castelli, Giordana, Sugoi, and virtually every other European Jersey. Pearl Izumi I wear a Large. All fit me very snug as I only buy race fit jersies.
I have only worn this on a mountain ride. The tempature was 45F and we had some sun. I had a jersey and a think base under the jacket and it was very warm. It ended up zipping off the sleeves at the top of the first 24 min clime. The zip off feature is very cool especially that it gives you some sleeve vs just acting as a vest. Decending at 20+mph showed the wind breaking capabilities of the jacket. I didn't have the sleeves on and my arms were cold and my torso toasty. I highly recommend the jacket.