I've just finished building up this frame (although it is the Tantan GR048 with integrated cables). The quality of the frame looks good, and the headset area is uniform. The bearings fit securely without being difficult to press in. The frame also looked good on the inside, very smooth with no crinkles visible. Assembly went smoothly, but routing a mechanical groupset through the handlebars would've been a nightmare; even with Di2 it was tight in some spots. The full build with bottle cages, computer mount and pedals is 9.1kg. With a lighter saddle and removing the many heavy screws in the frame, it should go down to around 8.9kg
I've only went on one ride, but first impressions are good. Due to the longer wheelbase, slack steering angle and low BB, it feels very planted and stable on gravel, especially on descends. On the road it almost feels too stable though, coming from a more CX based frame (GR029). Definitely less agile and slower turning. The frame itself feels stiff and efficient, and the handlebars feel much stiffer than my previous Deda Superzero stem.