Magnetite and gallium substituted cuboferrites with a composition of GaxFe3−xO4 (0 ≤ x ≤ 1.4) were fabricated by thermal decomposition from acetylacetonate salts. The effect of Ga3+ cation substitution on the structural and thermomagnetic behavior of 4–12 nm sized core-shell particles was explored by X-ray and neutron diffraction, small angle neutron scattering, transmission electron microscopy, Mössbauer spectroscopy, and calorimetric measurements. Superparamagnetic (SPM) behavior and thermal capacity against increasing gallium concentration in nanoferrites were revealed. The highest heat capacity typical for Fe3O4@Ga0.6Fe2.4O4 and Ga0.6Fe2.4O4@Fe3O4 is accompanied by a slight stimulation of fibroblast culture growth and inhibition of HeLa cell growth. The observed effect is concentration dependent in the range of 0.01–0.1 mg/mL and particles of Ga0.6Fe2.4O4@Fe3O4 design have a greater effect on cells. Observed magnetic heat properties, as well as interactions with tumor and healthy cells, provide a basis for further biomedical research to use the proposed nanoparticle systems in cancer thermotherapy (magnetic hyperthermia).