I've done a fair bit of manual testing against the Gamma() function in Maple 2021, and it seems to be OK. Graphing the output is tricky because 1) the output gets very large very quickly (by definition), and 2) the really interesting range exceeds the floating-point capability of Maple, which is how Maple draws graphs. Other checks include graphing the absolute difference, the absolute proportional difference, and the logs of these, which are easier to see. All of these look OK.