Gamow-Teller (GT) transitions are the most common weak-interaction processes in the Universe. They play important roles in various processes of nucleosynthesis, for example, in the rapid proton-capture process (rp-process). In the pf-shell region, the rp-process runs through neutron-deficient nuclei with T-z = -2, -1, and 0 mainly by means of GT and Fermi transitions, where T-z is the z component of isospin T defined by T-z = (N = Z)/2. Under the assumption of isospin symmetry, mirror nuclei with reversed Z and N numbers, and thus with opposite signs of T-z, have the same structure. Therefore, symmetry is also expected for the GT transitions starting from and ending up in mirror nuclei. We have been studying the T-z = -2 -> -1 and -1 -> 0 GT transitions in beta decays, while those from stable T-z = +2 and +1 nuclei by means of hadronic (He-3; t) charge-exchange (CE) reactions. The results from these studies are compared in order to examine the mirror-symmetry structure in nuclei. In addition, these results are combined for the better understanding of GT transitions in the pf-shell region.