In violent neutrino-induced reactions at the core-collapse stage of type II supernovae, Gamow-Teller (GT) transitions starting from stable as well as unstable pf-shell nuclei play important roles. In the β-decay study of these unstable pf-shell nuclei, half-lives can be measured rather accurately. On the other hand, in high-resolution (3He, t) charge-exchange reactions at 0°, individual GT transitions up to high excitations can be studied. Assuming the isospin symmetry for the strengths of Tz = ±2 → ±1 analogous GT transitions, we present a "merged analysis" for the determination of GT transition strengths starting from proton-rich Tz = -2 nuclei. We applied this analysis to the A = 52, T = 2 system, and it was found that the GT transitions and the properties of the 52Ni β decay can be understood better by combining the mirror GT strength distribution obtained from the 52Cr(3He, t) reaction.