The substitution of numerous cations into hexagonal ferrite has been extensively used to endow novel properties and functionalities for various applications. In the present work Gd-Tl substituted barium hexaferrites prepared by co-precipitation process, having the composition Ba0.75Cu0.25(GdxTl0.5-x)Fe11.5O19 (x = 0.0, 0.25 and 0.50). The hexaferrite formation during calcination of sample x = 0.25 was confirmed by TGA/DSC which was processed at 1000 degrees C for 3 h. The analysis of X-ray diffraction depicts the existence of magneto-plumbite structure with the formation of a minor secondary alpha-Fe2O3 phase x <= 0.0 and BaFe2O4 phase x <= 0.50. UV-Vis spectra reveal the dropping down behavior in the optical energy band gap from 2.47 eV to 1.74 eV. The grains with hexagonal platelet-like shape having size of 0.415-0.446 mu m of magnetic powder nanoparticles (MPs) are observed by SEM images. The energy dispersive spectrometer (EDS) analysis was employed for presence of ferrite elements within a single particle. Hysteresis loops signifies the magnetization (M-s) and remnant magnetization (M-r) first increases up to x = 0.25 then reduces with the substitution (x) increment; contrarily, the coercivity (H-c) exhibited initially decreased with maximum content of Tl at x = 0.0 then increases at x = 0.25 after that it decreases at x = 0.50. Maximum values such as M-s (51.727 emu/g), M-r (28.061 emu/g), and H-c (4.057 kOe) are attained for x = 0.25 at room temperature. The synthesized magnetic nanoparticles are found to be suitable for microwave absorbing materials, permanent magnets, catalyst, high density recording media and optoelectronic devices.