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Sørensen-Mølmer gate and Milburn gate are two geometric phase gates, generating nonlinear self-interaction of a target mode via its interaction with an auxiliary mechanical mode, in the continuous and pulsed interaction regime, respectively. In this paper, we aim at unifying the two gates by demonstrating that Sørensen-Mølmer gate is the continuous limit of Milburn gate, emphasising the geometrical interpretation in the mechanical phase space. We explicitly consider imperfect gate parameters, focusing on relative errors in time for Sørensen-Mølmer gate and in phase angle increment for Milburn gate. We find that, although the purities of the final states increase for the two gates upon reducing the interaction strength together with traversing the mechanical phase space multiple times, the fidelities behave differently. We point out that, the difference exists because the interaction strength depends on the relative error when taking the continuous limit from the pulsed regime, thereby unifying the mathematical framework of the two gates. We demonstrate this unification in the example of an optomechanical system, where mechanical dissipation is also considered. We highlight that, the unified framework facilitates the new method of deriving the dynamics of the continuous interaction regime without solving differential equations.