The gains for the three 4d→4p lasing transitions in Ni-like Ta collisional excitation X-ray lasers are studied in this paper. The study shows that gain and inversion at 4.483 nm and 5.097 nm lines are mainly contributed by collisional excitation from the ground state of Ni-like ion, while gain and inversion at 7.442 nm are mainly contributed by both collisional excitation from the ground state of Ni-like ion and recombination from the groud state of Co-like ion. In addition, the ionization rates from Cu-like ion and Ni-like ion in the important region are of the same order, which leads not only to overionization but also to low fractional population of Ni-like ion. The important role of autoionization and dielectronic capture in the ion population is also discussed. Moreover, the effect of resonance lines reabsorption is studied by escape factor method. Finally, as a result of study, the optimum region for gain in Ne/Te plane at 4.483 nm laser line is: Te≈1～1.5 keV, Ne≈(1～4)×1021 cm-3.