Guanidinium nitroprusside GNP, (CN3H6)2[Fe(CN)5NO] has been investigated in single-crystalline form by nuclear resonant forward scattering (NFS) using synchrotron radiation (Mössbauer spectroscopy in the time domain). This method provides a direct measure of effective absorber thickness and therefore also of the Lamb-Mössbauer factor fLM. GNP has the advantage that all [Fe(CN5)NO]2- anions are practically aligned within the crystal. For the two different crystal orientations, with the crystallographic a- and c-direction parallel to the synchrotron beam, respectively, we have obtained f(a)LM = 0.122(10) and f(c)LM = 0.206(10), i.e., GNP exhibits significant anisotropic vibrational behavior. The quantum beat pattern of the NFS spectra obtained for the two different crystal orientations is discussed on the basis of radiation characteristics of the polarized synchrotron beam and the multipole transitions of oriented 57Fe nuclei.