The formula given in de Haan and Ferreira, 2007 (Springer). Note that the latter differs from that found in Drees, Ferreira and de Haan.
gpd.abias(shape, rho)
a vector of length containing the bias for scale and shape (in this order)
shape parameter
second-order parameter, non-positive
Dombry, C. and A. Ferreira (2017). Maximum likelihood estimators based on the block maxima method. https://arxiv.org/abs/1705.00465