Selective product crystallization for separation and recycling of homogeneous catalysts represents one novel promising method to develop sustainable chemical processes. This specific application holds two main challenges: (1) In the crystallization and subsequent filtration and washing process, isolation of a pure crystalline product is required to obtain the maximum quantity of catalyst in the filtrate for recycling, and (2) the entire process must be operated in an inert atmosphere to maintain the catalytic activity of oxygen-sensitive species. Key to the isolation of a pure product is precise control of the crystallization process, which is realized in this work by gassing-induced nucleation using inert argon. Assuming that the nucleation step is crucial for the control of particle properties of the crystalline product and the related efficiency of downstream separation by filtration and washing, we systematically investigated the effect of controlled nucleation by gassing on these quantities using the oleo-chemical model system, 1,12-dimethyl dodecanedioate/methanol. The experimental results verify that by gassing with argon, the nucleation step can be initiated at lower supersaturation compared to uncontrolled cooling crystallization with spontaneous nucleation despite the comparably small metastable zone width of the investigated model system in the range of 1–3 K. It is further demonstrated that consequently less nuclei are formed that grow to larger product crystals, which can be more efficiently separated from the mother liquor in the filtration and washing step, reducing the mother liquor leaching to the isolated product to nearly 100 ppm. Finally, it is verified that gassing crystallization with argon offers a considerable benefit for the isolation of a pure crystalline product while maintaining inert operating conditions required for recycling of oxygen-sensitive catalysts.