The gallium–indium eutectic alloy is in a focus of modern studies because of its great potential for microelectronics, wearable electronics, electronic skin, medical diagnosis, and soft robotics. Novel applications of the Ga–In alloy require better understanding of the effect of size reduction on its properties. Here we report the NMR studies of the nanostructured Ga–In alloy within pores of the opal matrix. We demonstrated the stabilization of the segregated gallium-rich crystalline phase with a structure of β-gallium, this phase being metastable in bulk. The temperature dependences of the NMR isotropic shift for gallium nuclei within a temperature range from 11 K up to confined alloy melting and of the quadrupole constants for two gallium isotopes were also obtained. The experimental results suggested that only the gallium-rich phase with a structure of β-Ga crystallized under opal nanoconfinement.