Trivalent rare-earth (RE³⁺ = Eu³⁺, Tb³⁺, Dy³⁺, and Sm³⁺) ions activated Gd₂O(CO₃)₂·H₂O chrysanthemum-like flowers are prepared by a modified urea-based homogeneous precipitation via a template free hydrothermal synthesis route. Subsequently, Gd₂O₃ monodispersed spheres were obtained after calcining at 750 °C. The growth mechanism of the Gd₂O(CO₃)₂·H₂O:RE³⁺ chrysanthemum-like morphology (homogeneous precipitation) and their transformation to monodispersed spheres (heterogeneous nucleation) are established by taking scanning electron microscope and transmission electron microscope images of the intermediate products. The thermogravimetric analysis, Fourier transform infrared analyses confirmed the decomposition of CO₂ and OH groups, and the corresponding XRD patterns exhibited the Gd₂O(CO₃)₂·H₂O and cubic Gd₂O₃ phases. The photoluminescence measurements are used to explore the emission behavior of different RE³⁺ ions activated Gd₂O₃ spheres. The Gd₂O₃:Eu³⁺ shows gorgeous red emission with high purity red color as compared to the commercial Y₂O₃:Eu³⁺ phosphors. Gd₂O₃:Tb³⁺, Gd₂O₃:Dy³⁺ and Gd₂O₃:Sm³⁺ exhibit green, yellow and rich orange emissions, respectively. The Tb³⁺/Eu³⁺ co-doped sample shows warm white light by controlling the energy transfer. At minimal parameters, the cathodoluminescence intensity of Gd₂O₃:Eu³⁺ is beyond the experimental limit for 5 kV of accelerating voltage. The CIE chromaticity coordinates were also calculated from the PL and CL spectra of RE³⁺ ions to establish their color richness.