How does this jacket differ from the Gamma AR? Similar fit and use? Thanks

Well yes and no. Similar fit, different uses really. The Polartec Powershield O2 (as in oxygen, get it?) in the Griffon is a new fabric that uses a higher loft backing and a slightly looser shell weave, for more warmth and more breathability, a trade off for less wind and water resistance. The old Gamma AR design is a much tougher shell that is less warm, and would be better for climbing, alpinism, and and anywhere where less warmth and breathability and better weather and abrasion resistance would come in handy. The Griffon excels at being a cold weather low-activity midlayer, or a high-activity outer layer i.e. doing ski touring or splitboarding laps. Hope that helps.

Among other things, O2 is more malleable (less rigid) than regular Powershield. It is also less windproof but more breathable for better use as a midlayer.

Both are pretty much the same material, Polartec Powershield except the Griffon has Polertec Powershield 02. The main difference there is a resistance to pilling and the outside is very smooth with the 02 variation. Fit is very similar between the two and both are about waist length. Gamma AR features : Highly breathable warmth• Wind and moisture resistant• Waist length with drop back hem• No-lift gusseted underarms• Full front zip with interior wind flap• Laminated hem drawcord• Two hand pockets• One internal chest pocket• Moulded Zipper Garages• Soft brushed lined collar• DWR finish Griffon features: • Double layer of fleece on collar• Two hand pockets with laminated zippers• One sleeve pocket with laminated zippers• Articulated elbows• No-lift gusseted underarms• Stretch knit gusseted cuffs• Laminated hem drawcord