I actually have the Gan 105, which uses a slightly heavier carbon than the Gan S. CC special ordered it for me.
My review might be best understood in terms of the sort of bike I was looking for as well as contrasted to my previous bike. My last bike was a 2009 Cervelo S2. Based on my particular strengths as a racer, I preferred an aero road bike. The S2 seemed to fit the bill, but as I rode it more I began to see the defects of the first generation aero designs. Because of the narrow and deep airfoil shapes everywhere, the bike was stiff in all the wrong places and compliant in the wrong places. During hard efforts (particularly climbs) I could even see the chainring deflect laterally, which meant I was losing power. Plus the bike was twitchier than I would have liked and, worse, difficult to control in crosswinds. If I got a new bike, I wanted these defects fixed if possible.
The Gan ticked all of the boxes for what I wanted. First, it is beautiful. Seriously. The pictures don't do the bike justice. Put a set of Zipp or ENVE aero road bars on and it looks deadly and elegant like a racing bike should.
As a latest generation aero road bike, it adopts the kamm tubing shape that most other brands now use. I can't quantify the aerodynamics, but Pinarello did a white paper on the almost identical F8 and they assert that the bike is invisible to drag until the air reaches the bottles and bottom bracket. Taking a rider into account, I think Pinarello said a rider goes about 6% faster on this bike versus the Dogma 65.1 for the same power. Considering that the frame is only about 30% of the drag, that's not bad. How it compares to other brands I have no idea. A nice touch is the aero stem and spacers. The seatpost clamp, a wedge you can tighten with a hex wrench, is invisible to the wind and very easy to use. There is no rear wheel cutout, but that's often more problem than it's worth since rocks often get jammed in there. Plus the kamm seat tube probably makes it unnecessary.
I can say that the bike handles so much better in heavy crosswinds than my old S2. The S2 was terrifying to ride in those situations. The Gan's geometry, with a lower bottom bracket and longer chainstays than the average race bike, make the Gan very smooth and stable. This may not be to everyone's preference, but I happen to like it. The tubing, helped by the asymmetric seat and chainstays, is rock solid. The amount of flex is slight enough that I can no longer see it. As a result, this bike climbs like a beast. The fork is massive and aerodynamic and it bows outward, which seems to dampen road vibration. The S2, in contrast, transmitted a lot of chatter. That, combined with twitchiness in the steering, made things rather fatiguing after a while. I did my own internal cable routing for the Gan--it's easy, especially if you have a bit of casing from some housing to guide the cables.
There are other options out there for aero bikes, and all are pretty good. This bike stands out for its looks, which are distinctive, the fork, and its geometry. It's a comfortable and confident ride.