Gallium ferrite, GaFeO 3 (GFO), has ferrimagnetic (FM) and piezoelectric orderings and has been intensively studied recently for its potential application as a magnetoelectric ferrimagnet [1]. Site-disorder in bulk GFO is reported to be induced by adopting different preparation conditions and methods, which are shown to affect the magnetic properties significantly [1]–[3]. Moreover, the physical properties of GFO at reduced grain sizes remain underexplored due to the challenges associated with their synthesis with a proper control over the crystalline phase. In addition, the origin of the doping effects on the correlation between the site-disorder and magnetism in the nanosized GFO system has not yet been completely established. Therefore, it is worthy to explore the possibility of tuning the magnetic properties of doped GFO nanoparticles. In this work, we prepare a series of Ga 1-x Mg x FeO 3 (GMFO x ) $(0\leq \mathrm{x}\leq 0.1)$ nanoparticles by a modified Pechini method to systematically study the effect of Mg-doping on their structural, magnetic, and dielectric properties. Interestingly, the magnetization and electric polarization of the doped samples are enhanced compared to those of the pristine sample of GFO. The magnetic hysteresis curves measured below the FM transition temperature $(\mathrm{T}_{\mathrm{C}})$ exhibit two-phase-like magnetic behavior consisting of soft and hard magnetic phases. The results could be explained by the site-disorder of the cations in GMFOx. Furthermore, thermo-magnetic irreversibility and cusp in zero-field cooled magnetization are observed for all the GMFOx nanocrystals, which are attributed to the inherent magnetic anisotropy. Moreover, variable temperature dielectric measurements show an anomaly in the dielectric constant at the temperature close to the $\mathrm{T}_{\mathrm{C}}$ indicates the magnetoelectric coupling of the GMFOx nanocrystals. These results open wide perspectives for the room temperature FM magnetoelectric GFO ferrite in spintronic devices.