The aim of this study was to better understand the incorporation of rare-earth elements in glass-ceramics of nuclear interest. We synthesized glass-ceramics from glasses in the system SiO2–B2O3–Na2O–CaO–Al2O3–MoO3–Gd2O3 by various heat treatments. Gadolinium is used both as a spectroscopic probe and as a minor actinide surrogate. Glass-ceramics contain only one crystalline phase in the bulk: powellite (CaMoO4). This phase can incorporate Gd3+ and Na+ ions by substitutions on the Ca site. We demonstrated that the charge compensation by Na+ favors the incorporation of rare-earth elements. Moreover, the incorporated elements do not seem to be randomly distributed into the powellite structure.