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.
Steve Kargl sent me his libm testing code, which I used to verify
that the excpetions for certain key values were correct. Testd in
this case were +-inf, +- NaN, +-1 and +-0.