Detailed measurements of the low-temperature infrared absorption of tellurium- and sulfur-doped n-type gallium phosphide have been performed. The set of well-characterized samples with neutral-donor concentrations ranging from 1.5×1017 to 6.4×1018 cm-3 allowed us to follow the concentration dependence of the observed absorption band. The spectra have been described quantitatively, including the absolute value of the absorption coefficient. The contributions of transitions from the donor ground state to the different final states have been separated. No effect of the clustering of donors has been observed on the photoionization part of the absorption. The line-shape of the transition to the resonant state has been deduced. Its width and its energy position are analyzed in terms of Fano-type interaction with the X1-band continuum.