Minor Description
In the gamma method, when handling the case "absX > 20", the computation of gammaAbs should replace "x" (see code below with x in bold) by "absX".
For large negative values of x, the function returns with the wrong sign.
final double gammaAbs = SQRT_TWO_PI / x *
FastMath.pow(y, absX + 0.5) *
FastMath.exp(-y) * lanczos(absX);