The guanidinium methanesulfonate is a layered self-assembled organic crystal which presents a bilayer structure and two different (001) planes can be exposed upon cleavage. In a previous paper we have shown that the analysis of the rate of decay in the thermal attenuation of the diffraction intensities in the scattering from the (001) surface of single crystal guanidinium methanesulfonate allows for distinguishing between the two possible (001) cleavage planes. However, since a more detailed knowledge of the He-surface interaction was lacking, in this paper we complete the investigation by studying the attractive part of the interaction potential between He and the surface terminated by methyl groups projecting outward from the hydrogen bonded sulfonate and guanidinium sheets. Four bound state levels have been determined and a well depth D=(6.7+/-0.2) meV has been estimated. (C) 2003 American Institute of Physics.