The solid solubility of Gd in (Mg) is studied experimentally using microstructure, chemical and EDX analysis on permanent mould chill cast samples which were annealed for 1-14 days at 300-550 °C. Based on those key data and all the critically assessed experimental phase equilibrium and thermodynamic data of the Mg-Gd system two Calphad-type thermodynamic descriptions are developed. This provides calculated phase diagrams, and an inherent inconsistency between the vapor pressure data and the Mg-rich phase diagram is indicated. A previous misperception of ''experimental'' enthalpy of formation data of binary GdxMgy intermetallic compounds is also revealed. The assumption of systematic trends in these key thermodynamic data for the series of rare earth elements R = (La, Ce, Pr, Nd, Sm, Gd) is scrutinized.