Abstract We perform a full nuclear-network numerical calculation of the r -process nuclei in binary neutron-star mergers (NSMs), with the aim of estimating gamma-ray emissions from the remnants of Galactic NSMs up to 10 6 yr old. The nucleosynthesis calculation of 4070 nuclei is adopted to provide the elemental composition ratios of nuclei with an electron fraction Y e between 0.10 and 0.45. The decay processes of 3237 unstable nuclei are simulated to extract the gamma-ray spectra. As a result, the NSMs have different spectral colors in the gamma-ray band from various other astronomical objects at less than 10 5 yr old. In addition, we propose a new line diagnostic method for Y e that uses the line ratios of either 137m Ba/ 85 K or 243 Am/ 60m Co, which become larger than unity for young and old r -process sites, respectively, with a low -Y e environment. From an estimation of the distance limit for gamma-ray observations as a function of age, the high sensitivity in the sub-megaelectronvolt band, at approximately 10 −9 photons s −1 cm −2 or 10 −15 erg s −1 cm −2 , is required to cover all the NSM remnants in our Galaxy, if we assume that the population of NSMs by Wu et al. A gamma-ray survey with sensitivities of 10 −8 –10 −7 photons s −1 cm −2 or 10 −14 –10 −13 erg s −1 cm −2 in the 70–4000 keV band is expected to find emissions from at least one NSM remnant under the assumption of an NSM rate of 30 Myr −1 . The feasibility of gamma-ray missions observing Galactic NSMs is also studied.