I purchased the Gamma LT to replace 2 aging softshell jackets - an ancient Gamma SV and a Patagonia Super Guide. I've now used the LT a few times and have some initial impressions.
Fit. I'm a solid Large in Patagonia tops, but thanks to the Arc'teryx size break (Large ends at 43" chest) jacket fit is always questionable - Large tends to be a snug fit, but I swim in XL. My Gamma SV is a Large and is a snug fit, not really allowing even a second layer beneath it. A nice athletic cut though, and great sleeve length. Based on my Arc'teryx experience I ordered an XL Gamma LT, and it too is oversized. Unlike the Gamma SV, the new jacket has a squarer cut and room for layering (good), but is also rather baggy (not so good). The body is way long and so are the sleeves. I plan to use the jacket for traveling and around-town use, so body length isn't that bothersome. The sleeve length IS, and I'll have to figure out what to do about that. In an ironic twist, even these excessively long sleeves leave my wrists exposed when I reach overhead because the fit isn't snug to the wrist and there's no way to adjust the fit.
Features. The LT has two large chest pockets and a pocket at the left bicep, all exterior. The old SV has two large exterior chest pockets and an internal pocket. The SV chest pockets are especially high and rotated forward. These actually work with a harness or pack hip belt, where the LT's pockets are more likely to get caught underneath. That is also the problem with the pockets on the Patagonia jacket. However, I still like the Patagonia jacket's pocket arrangement - two exterior side pockets and an exterior vertical pocket at the left breast, plus an internal chest pocket. The Gamma LT doesn't seem to have enough pockets for my purpose, thought the chest pockets are HUGE. They're also made of a mesh, so partly open pockets can serve as vents.
Otherwise, the bottom hem drawcord on the LT is probably the easiest to adjust of the 3 jackets. The LT has a dropped tail, more so then either of the other jackets. The LT's collar, when zipped up is a good height and fit, and it's lined with a soft micro fleece for comfort. The main zipper for the Gamma LT is a sturdy toothed zipper. Runs well, doesn't snag. The characteristic Arc'teryx rubbery draft flap is a bit smaller than that on my Gamma SV, and so doesn't get in the way as much.
Fabric. I like stretch-woven softshell jackets for travel and around-town use because stretch-woven fabrics are so versatile: sturdy, breathable, wind- and water-resistant, food-resistant, and stretchy. I dislike softshell jackets for actual outdoors use because they tend to be both heavy and bulky; unless I'm sure I'll be wearing the jacket all the time I don't want it along. That's certainly true for the Gamma SV's Powershield fabric, and mostly true for the stretch-woven used in the Super Guide. The Gamma LT's fabric, called Burly Double Weave, is a bit different: smooth on the outside, subtly barred on the inside, and lighter than the fabric on either of the other jackets. Also softer. Unlike the other two jackets, which never got used for climbing, skiing, etc. I WOULD use the Gamma LT for those things - if the fit was better. Wind resistance is on par with the other jackets. Breathability seems good. Haven't had enough moisture to say anything about water resistance.
Overall, this is a nice jacket. My main problem with it is really about where I fit in the Arc'teryx sizing scheme. The product fit is true to that described in the bird's sizing charts; I just don't fit well into that sizing. Materials and features seem to work pretty well.