Starting with the stoichiometric and highly homogeneous gel-precursor, single-phase metastable melilite La₂Ga₃O_7.5, as the end-member of solid solution La_1+xSr_1−xGa₃O_7+x/2 (0 ≤ x ≤ 1), has been synthesized by solid-state reaction at 700 ℃ for 2 h via a kinetically favorable mechanism and characterized by X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), AC impedance spectroscopy, etc. It has been revealed that the as-synthesized melilite La₂Ga₃O_7.5 shows an orthorhombic symmetry with crystal cell parameters a = 11.4690(1) Å, b = 11.2825(4) Å, and c = 10.3735(4) Å, while has more Raman active modes than LaSrGa₃O₇ with a tetragonal structure, which was also synthesized under the same conditions for comparison, but tends to slowly decompose into perovskite LaGaO₃ and Ga₂O₃ when annealed at 700 ℃ for over 20 h driven by its meta-stability. Moreover, the metastable La₂Ga₃O_7.5 shows a higher XPS binding energy for the excess oxide ions in the crystal structure than those at normal lattice sites. Its intrinsic grain oxide ion conductivity can reach as high as 0.04 and 0.51 mS·cm^–1 at 550 and 700 ℃, respectively, characterized by a simple Arrhenius relationship ln(σT)–1/T with invariable activation energy, E_a = 1.22 eV, over the temperature range from 300 to 700 ℃, along with an apparent grain boundary conductivity that is about double that from the grains thanks to the clean grain boundaries. This paper provides a new strategic approach to the synthesis of complex oxides that may be of high performance but are difficultly achieved by the conventional ceramic method at high temperatures.