The copolymerization of styrene with divinylbenzene (DVB) in the presence of silica particles produces a commercial silica-supported polymer (SiO2-P) that was chemically reacted with acrylonitrile to synthesize silica-supported polyacrylonitrile (PAN/SiO2). PAN/SiO2 was functionalized by grafting amidoxime moieties to produce PAO/SiO2 sorbent (silica-supported polyacrylamidoxime). The sorbent (PAO/SiO2) was characterized by XPS, TGA, FTIR, pHpzc, elemental and SEM-EDX analyses and tested for Ga(III) recovery from aqueous solutions. The sorption performance was investigated through the study of pH effect, sorption isotherms at different temperatures, and uptake kinetics under slightly acidic conditions (around pH 4) and at pH close to 13.7 (similar to Bayer liquor). The equilibrium was reached within 60 min; the kinetic profiles can be fitted by the pseudo-second order rate equation (PSORE) and the resistance to intraparticle diffusion equation (RIDE). The maximum sorption capacity reaches 1.34 mmol Ga g−1 at 25 °C (1.76 mmol Ga g−1 at 55 °C): the sorption process is endothermic for solutions prepared by alkaline dissolving of Ga2O3. The sorption capacities are higher for solutions prepared with Ga(NO3)3 salt: the maximum sorption capacity increased up to 2.1–2.6 mmol Ga g−1 (from 25 to 55 °C). Langmuir and Sips equations were used for fitting sorption isotherms. Metal desorption and sorbent recycling were performed using 1.5 M HCl solution. Forty-five minutes were sufficient for achieving complete desorption of Ga(III). Sorption and desorption performances remain stable for a minimum of 5 cycles. Sorbent selectivity for gallium recovery was demonstrated in the presence of an excess of competitor cations. The sorbent was successfully applied for Ga(III) recovery from Bayer liquor at high pH values (around 13): combining relatively high efficiency and selectivity compared with LSC-600 amidoxime resin.