An improved gastric bypass prosthetic (GBP) device and an improved GBP fixation device, as well as a GBP kit for the surgical treatment of obesity. In one form, a gastric bypass prosthetic (GBP) device of the invention includes a optional flexible tube extending along a central axis from a proximal end to a distal end, and having a tubular tissue fixation portion at or near said proximal end. The tissue fixation portion extends along the central axis between a proximal end and a distal end of the tissue fixation portion, and has an annular outer surface between the proximal end and the distal end of the tissue fixation portion.