According to experimental data at GSI, the rates of the number of daughter ions, produced by the nuclear K shell electron capture decays of the H-like heavy ions with one electron in the K shell, such as ;{140}Pr;{58+}, ;{142}Pm;{60+}, and ;{122}I;{52+}, are modulated in time with periods T_{EC} of the order of a few seconds, obeying an A scaling T_{EC} = A/20 s, where A is the mass number of the mother nuclei, and with amplitudes a_{d};{EC} approximately 0.21. We show that these data can be explained in terms of the interference of two massive neutrino mass eigenstates. The appearance of the interference term is due to overlap of massive neutrino mass eigenstate energies and of the wave functions of the daughter ions in two-body decay channels, caused by the energy and momentum uncertainties introduced by time differential detection of the daughter ions in GSI experiments.