The Gwen avocado, grown by Will Brokaw in Soledad, is a little bigger & more round than a Hass.  Its thick yet slightly less pebbly skin ripens green. Chefs love their ease of use; they ripen evenly & predictably and the flesh maintains a bright green, just-cut-into color for a surprisingly extended period of time. And they have a thick, buttery texture and a noticeably pleasant avocado flavor.
Genealogically, they are a Hass grand daughter. In a UC Riverside variety trial, the original Thille Avocado tree was grown from a Hass seed. Then a Thill seed eventually grew into the original Gwen tree. The responsible professor, Grey E Martin, named the Gwen after his wife.