In the plane wave spacetime, when gravitational waves pass by, an angular deviation exists between two free-falling gyroscopes, which naturally corresponds to the velocity variation. In the plane wave spacetime, with linear polarization taken into account, no contribution is made by the first-order terms of the initial separation distance $L$ (or the initial separation velocity $v_0$) and $\mathit{P}$ (or $\mathit{M}$) of two free-falling gyroscopes to the memory effect, while contributions are initiated from the second-order terms. Under certain circumstances, the second-order contribution of the initial separation distance $L$ is of the same order of magnitude as the first-order contribution of the initial separation velocity $v_0$. When $\mathrm{both}+\text{polarization}$ and $\times$ polarization are taken into account, in the context of the merger of supermassive black holes and with the initial separation velocity approaching the speed of light, the order of magnitude of the angle is ${10}^{-16}\mathrm{rad}$.

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• General relativity