I have implemented a preliminary version of
\fp_factorial:n and \fp_gamma:n
The current working precision of \fp_gamma:n is limited to 1e-11.
So far, error checking for the argument of factorial is missing and I
don't know how to integrate this into l3fp, especially making these
available in \fp_eval:n as factorial(x) and gamma(x).
If anyone would be so kind as to provide some guidance, I would be