The influence of composition and well-width fluctuations on the optical gain in (In, Ga)N multiple quantum wells for different barrier dopings and compositions is studied under quasistationary conditions. Systematic temperature- and excitation-density-dependent gain measurements were performed by means of the variable-stripe-length method. The correlation between optical properties such as gain or temperature behaviour of the laser threshold and the crystal quality of the samples is demonstrated. In addition, by means of photoluminescence measurements, the temperature dependence of the quantum efficiency is determined, which gives information on the activation processes to nonradiative recombination channels. Thermal activation energies obtained from these measurements can be correlated with the depth of potential minima caused by composition fluctuations in the quantum well. The experimental temperature-dependent threshold density and gain can be successfully explained by use of a simple model of band-to-band transitions considering localization effects.