In the search for the nuclide with the largest probability for neutrinoless double-electron capture, we have determined the Q_εε value between the ground states of Gd152 and Sm152 by Penning-trap mass-ratio measurements. The new Q_εε value of 55.70(18) keV results in a half-life of 1026yr for a 1 eV neutrino mass. With this smallest half-life among known 0νεε transitions, Gd152 is a promising candidate for the search for neutrinoless double-electron capture.