ToMatrix

M = ResponseFunction.ToMatrix(G) returns a matrix representation of $G$ such that $$ G(\omega,\Gamma) = A_0 + B_0^* \left( \frac{1}{(\omega+\mathrm{i}\Gamma/2) - M} \right)_{[1..Blocksize,1..Blocksize]} B_0^T$$

As we only need the left top matrix of dimension Blocksize of the inverse of the matrix the matrix $M$ is not uniquely determined. The exact form of $M$ depends on the type used for the response function.