Fundamental to investigating the coexistence of superconductivity and magnetic ordering in GdSr2RuCu2Oy (Gd1212) is the fabrication of macroscopic crystallographically oriented samples. To achieve this, we need to identify the conditions in which Gd1212 incongruent melting reaction, producing solid GdSr2RuO6 (Gd1210) in Cu-rich liquid, dominates over decomposition due to RuO2 sublimation. Only in these conditions is it possible, indeed, to grow oriented samples by any melting and resolidifying technique. Moreover, the optimal mixture of Gd1212 and its melting products has to be identified to perform melt-textured or flux-flow growth. By means of thermogravimetric and differential thermal analysis (TG–DTA) we analyzed, up to 1200 C in air, several mixtures of Gd1212 with Gd1210 or CuO, performing a scan of the entire CuO–Gd1212–Gd1210 coexistence line of the Gd–Sr–Ru–Cu–O phase diagram. Gd1212 melting temperatures have shown a certain dependence on composition, decreasing about 30 C for high Gd1210 content samples and about 20 C for high CuO content samples with respect to pure Gd1212. A huge undercooling was observed for resolidification processes that were revealed to start at temperatures around 990 C. Measurements proved that in this range of temperatures in air, no mass is lost at the end of each melting process, so that they are not affected by RuO2 sublimation. A strong dependence on the atmosphere has also been observed for pure Gd1212, melting temperature decreasing by more than 80 C from 100% O2 atmosphere to He atmosphere (less than 10− 2% O2). Measurements revealed that high …