Gadolinium-substituted cobalt-nickel ferrite Co0.5Ni0.5GdxFe2-xO4 (0 <= x <= 1.0) nanostructures have been synthesized by hydrothermal approach which results more hydrophilic surface properties important for biomedical applications. Structural analysis by X-ray diffraction revealed the formation of a single-phase spinel ferrite for all samples and crystallite size is ranging from 13 to 28 nm. Lattice constant decreases with increasing Gd3+ ion concentration due to differences between ionic radii of Gd3+ and Fe3+. Morphological analysis by scanning and transmission electron microscopy indicated the shape transformed from agglomerated particles into rod-shaped with increasing Gd content. Fourier transform infrared analysis also correlated the presence of the spinel ferrite structure. Optical band gap measurement implied that band gap decreases with increasing Gd content. In order to determine magnetic properties of cobalt-nickel spinel ferrite nanostructures, isothermal magnetization measurements have been obtained at 300 and 15 K using vibrating sample magnetometer. Magnetic properties are strongly depending on Gd substitution ratio, which alters the crystallite size, cation distribution, and exchange interactions between octahedral and tetrahedral sites of nanostructures. Saturation magnetizations decreased with increasing Gd substitution at both temperatures since cation distribution at different sites and large lattice distortion caused by Gd3+ ion substitution. Due to complex relations between the shape anisotropy, crystallite size, grain boundaries, secondary phases, and increasing Gd content observed in Co0.5Ni0.5GdxFe2-xO4 nanostructures, coercivity results in different magnetocrystalline anisotropy behavior.