A free impinging-jets microreactor (FIJMR) was successfully used for the ultrafast co-precipitation of gadolinium and iron(III) hydroxides and subsequent production of fine GdFeO₃ nanoparticles via precipitate heat treatment in the air. The effects of the concentration of Gd³⁺ and Fe³⁺ ions (c = 0.001-0.1 mol/L), the temperature of the reaction solutions (t = 0-50°C), the flow rate of the solutions (Q_j = 100-400 mL/min) and the jets impingement angle (2θ = 90-180°) on the mixing mode and nanoparticle size were comprehensively analyzed. The impact of co-precipitation parameters was discussed in detail from both physicochemical and hydrodynamic points of view. It was shown that the FIJMR co-precipitation resulted in isometric GdFeO₃ nanoparticles with variable average size (<D> = 22.8-40.6 nm) and size distribution variance (FWHM = 19.7-32.2 nm). The optimal parameters were found to be c = 0.01 mol/L, t = 0°C, Q_j = 200 mL/min and 2θ = 90° and allowed to produce ultrafine GdFeO₃ nanoparticles (<D> = 22.8 nm) with narrow size distribution (FWHM = 20.5 nm), unified morphology, high visible-light absorption and superparamagnetic behavior. Based on these results, FIJMR-assisted co-precipitation was concluded to be an efficient and prospective method for the physicochemical design and production of advanced functional nanopowders.