Manipulating the structure of alkylgallium alkoxides with asymmetric N-heterocyclic carbenes (NHCs) has led to the formation of essentially “on” and “off” states of catalytic gallium centers in the ring-opening polymerization (ROP) of rac-lactide (rac-LA), and has allowed to rationalize the effect of the NHC on the activity and stereoselectivity of Me2GaOR(NHC) complexes. The reactions of dimethylgallium alkoxides with asymmetric NHCs, due to differently substituted nitrogens, resulted in the formation of Me2GaOR(SI(Dipp-Mes)) (OR = OMe (1), OCH2CH2OMe (2), OCH(Me)CO2Me (3)) and Me2GaOR(SI(Me-Mes)) (OR = OMe (4), OCPh2Me (5)) complexes (SI(Dipp-Mes) = 1-(diisopropylphenyl)-3-(mesityl)-imidazolin-2-ylidene, SI(Me-Mes) = 1-(methyl)-3-(mesityl)-imidazolin-2-ylidene). The X-ray analysis of 1, 2, 4, and 5 revealed a strong effect of NHCs on their structure, manifested by the orientation of NHC with respect to Ga–CMe and Ga–O bonds, and OR alkoxide group. The latter, represented by a dihedral O–Ga–CNHC–N angle, have allowed to define the cavity required for the interaction of lactide with gallium and alkoxide groups, and subsequent insertion of lactide into the Ga–Oalkoxide bond. Despite small differences between the cavities of 1 and Me2GaOMe(SIMes) (SIMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene), the structures of NHCs were responsible for their isoselectivities, of Pm-max = 0.83 and Pm-max = 0.78, respectively, in the ROP of rac-LA at −20 °C. The much smaller cavity of 4 led to a minor activity and lower isoselectivity (Pm-max = 0.56 at −20 °C). Further decrease of the activity of Me(O,CNHC)GaOR (6), possessing both small cavity and, in contrast to 4, restrained rotation of Ga–CNHC bond of the chelate ligand, has supported the effect of NHC on the structure and activity of Me2GaOR(NHC) complexes. DFT calculations of the energy profile of the consecutive ring-opening of two lactide molecules into Ga–Oalkoxide bonds of 1, 4, and Me2GaOMe(SIMes) have confirmed the effect of NHC on their stereoselectivity.