Bioactive glasses containing gold nanoparticles (AuNPs) have been synthesized via the sol−gel route using HAuCl4·3H2O as gold precursor. The formation process of AuNPs was studied as a function of the thermal treatment, which induces nucleation of Au particles and influences their nature, optical properties, shape, size, and distribution. The physicochemical characterization indicates that the sample treated at 600 °C presents the best characteristics to be used as a bioactive material, namely high surface area, high amount of AuNPs located at the glass surface, presence of micropores, and abundant surface OH groups. In the case of samples either aged at 60 °C or calcined at 150 °C, AuNPs just begin their formation, and at this stage the gel is not completely polymerized and dried yet. A thermal treatment at higher temperatures (900 °C) causes the aggregation of AuNPs, forming “AuMPs” (i.e., Au microparticles) in a densified glass−ceramic material with low surface area, absence of pores, and low number of surface OH groups. These features induce in the glass−ceramic materials treated at high-temperatures a lower bioactivity (evidenced by SBF reaction), as compared with that exhibited by the glass samples treated at 600 °C.