I use the Kolor AutoPano Giga panorama-stitching software, recently acquired by GoPro, but I have yet to produce a gigapixel panorama like those they pioneered. This brings up an interesting question: given a camera and lens, what would the pixel size of the largest 360° stitched panorama be?
Wikipedia to the rescue: using the formula for the solid angle of a pyramid, the full panorama size of a camera with m megapixels on a sensor of a x b using a focal length of f would be:
m * π / arctan(ab / 2f / sqrt(4f2 + a2 + b2))
For single-strip panoramas of height h (usually a or b), the formula would be:
m * π * h / 2f / arctan(ab / 2f / sqrt(4f2 + a2 + b2))
(this applies only to rectilinear lenses, not fisheyes or other exotics).
The only way I can break through the gigapixel barrier with a prime lens is using my 24MP APS-C Fuji X-T2 with a 90mm lens.
Now I could reach 10GP with my Nikon Z7 and the Nikkor 500mm f/5.6 PF.