(Sr_0.975Mn_0.02Ln_0.005)Ga₂S₄ (Ln = La and Ce) phosphor materials were synthesized by the sulfurization of oxide-carbonate precursors prepared using aqueous solution processes based on amorphous metal complex method or micro-gel freeze-drying method. As a group, multicomponent sulfide phosphors obtained from the amorphous metal complex precursors demonstrated stronger emission brightness than materials of the same composition synthesized by the micro-gel spray freeze-drying method. (Sr_0.975Mn_0.02Ce_0.005)Ga₂S₄ phosphor demonstrated blue emission intensity reaching 40% intensity of one of the best commercially available BAM phosphors excited by 408 nm light. (Sr_0.975Mn_0.02La_0.005)Ga₂S₄ red phosphor exhibited strong deep red emission when excited by 310 nm light, and in terms of emission intensity it outperformed Ba_1.59Sr_0.4ZnS₄:Eu²⁺. Nevertheless, the former material can not be excited by visible or near UV light.