The Kosterlitz-Thouless ͑KT͒ transition is investigated for gauge glass models in two dimensions by means of the nonequilibrium relaxation ͑NER͒ method. Two kinds of models, which have the same symmetry, are analyzed. Using the scaling analysis of the NER function on a large lattice with L = 1000, we confirm the KT transition numerically for both models. This indicates the stability of the KT phase against a small disorder, which was previously claimed by perturbation expansion and renormalization group arguments.The gauge glass ͑GG͒ model is a classical spin system with quenched disorder and has attracted much attention. It describes the thermodynamics of various systems such as disordered magnets with random Dzyaloshinskii-Moriya interaction, 1 Josephson-junction arrays with positional disorder in a magnetic field, 2 vortex glasses, 3 and crystal systems on disordered substrates. 4 In three dimensions, the spin-glass ͑SG͒ transition for a strongly disordered regime has been confirmed in the GG systems theoretically, 5,6 as well as experimentally. 7 In two dimensions, there is a controversy about the existence of the SG-like phase in the strongly disordered regime. Some numerical simulations suggested the quasi-long-range order, 8,9 while experimental observation 10 and other numerical simulations 6,11-14 deny it. Although the long-range SG order is denied in two dimensions following the Marmine-type argument, 15 there is a possibility of a phase in which the SG correlation decays in a power law. 16 In weakly disordered regime, there has been another controversy about the existence of the reentrant transition from the Kosterlitz-Thouless ͑KT͒ phase 17 to the non-KT one. Earlier works with real-space renormalization group ͑RG͒ analysis suggest reentrance. 1,2,18,19 The analysis has been modified and provides the absence of it. [20][21][22] The same results are obtained by Monte Carlo simulations. 4,6,9,13,23,24 and the RG analyses. 20,25 With all these studies, the instability of the KT phase against a small disorder is pointed out by the perturbation expansion and the RG analysis. 26,27 However, it is denied by numerical simulations 4,6,9,13 and other RG analyses. 9,20-22 Analytically, the gauge theory, which has provided several exact relations in Ising SG models, shows the absence of reentrance if the KT phase appears in a finite disordered regime. 16 The same result is also derived from a dynamical point of view obtained by the dynamical gauge theory. 28 While the results of gauge theory are plausible, it is necessary to assume the stability of the KT phase. Investigation of the phase diagram of these random XY models is still at a primitive stage as compared to the case of the Ising model.In the present study, we apply the nonequilibrium relaxation ͑NER͒ analysis to the GG models in two dimensions in order to clarify the stability of the KT phase against a small disorder. The NER method has been an efficient numerical technique for analyzing equilibrium phase transitions. It pro-vides the critical temperatu...