<p>Ferrites with the garnet structure containing rare-earth ions have very diverse magnetic properties, in particular due to different Landé <i>g</i>-factors of rare-earth ions and to the splitting of their energy levels under the effect of crystal fields and/or the spin–orbit coupling. Thulium iron garnet Tm₃Fe₅O₁₂ has a low gyromagnetic ratio. It has been shown in this work that the effective gyromagnetic ratio in such materials can be significantly increased (by a factor of 3–5) by diluting iron with gallium ions. In this case, the gyromagnetic ratio depends on both the content of gallium ions and their distribution between the octahedral and tetrahedral sublattices of the iron garnet. The possibility of achieving a high gyromagnetic ratio in ferrimagnets, which have no magnetic and angular momentum compensation points, has been experimentally discovered and theoretically confirmed for the first time. The gyromagnetic ratio is a key parameter determining the speed of processes in a magnetic spin system, and the results obtained in this work are important for a significant increase in the operation speed of spintronic devices based on ferrimagnetic materials.</p>