  We have obtained the general form of even mode potential function for the
gravitational perturbation of a static spherically symmetric spacetime.
Considering the minimal-spread Gaussian matter source, parameterized by a
smearing length scale {\Theta}, the associated quasinormal mode (QNM)
frequencies are computed numerically using the well known WKB formula. A
comparative study on the accuracy of the results based on different
approximation orders of the WKB formula indicates that the smearing parameter
{\Theta} has an upper bound, beyond which the spectra of odd and even modes
deviate from their usual isospectral nature.
