I have a vector $\mathbf{x}$ with a multivariate Gaussian distribution $$P[\textbf{x}\in S] =\int_{\textbf{x}\in S} \det(2\pi H^{-1})^{-1/2}\exp(-\frac{1}{2}\textbf{x}^TH\textbf{x})d\textbf{x}$$$$P[\textbf{x}\in S] =\int_{\textbf{x}\in S} \det(2\pi H^{-1})^{-1/2}\exp(-\frac{1}{2} \textbf{x}^T H\textbf{x}) \, d\textbf{x}$$How can I compute probability densities within the hyperplane $\textbf{x}\cdot \textbf{g}=C$ using the standard Lebesgue measure? ${}$
The closest question & answer I have found on this site was for a particular instance of Gaussian and hyperplane here.