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In this research, we are interested to study the physical properties of the double perovskite oxide Ba2GdNbO6 under the effect of octahedral tilting, where this material crystallises in the tetragonal structure (I4/m (no. 87), Z = 2). The structural, electronic and magnetic properties are investigated using the spin-polarised density functional theory within the framework of the GGA + U approximation. The structural stability for Ba2GdNbO6 compound has been verified; this shows that I4/m structure is reported as stable ground state of Ba2GdNbO6 compound. The phenomenon of octahedral tilting contributes to the stability of this material and to the apparition of the quantum aspect. The X-spectra study show that the structural transition can occur between the I4/m symmetry system (the stable ground state) and the Fm 3 ¯ m symmetry system (the excited state), the absorbed radiation of this structural transition corresponds to the Middle-Wave-Infrared (MWIR). The electronic structure of Ba2GdNbO6 compound has been investigated in the stable ground state; the results show that the octahedral Ba2GdNbO6 compound has semiconductor behaviour with an indirect band gap. The results of total and partial spin-magnetic moments of Ba2GdNbO6 compound show that the main contribution to the total magnetic moment comes from the Gd ion, it is obvious to note that the half-filled in Gd (4f7) orbital is responsible for the semiconductor behaviour. Moreover, the GGA + U approximation supported by QTL programme shows that the multiplicity in the occupied 4f-Gd states is corresponding to the lower energy level of (8 S ( 7 2 ) ↑ ). [ABSTRACT FROM AUTHOR]