The Gamow peak is a peak in the probability that atoms moving at some speed in a gas will fuse, i.e., that fusion will take place. The probability, called the Gamow factor, is the combination of two probabilities: the probability that two atoms will have sufficient relative speed (the Maxwell-Boltzmann distribution), and the probability of tunneling, a probability that quantum mechanics can provide. The probability is dependent upon the temperature of the gas. Tunneling is more likely the higher the speed, and given a temperature, there is a most probable speed, and above that, the faster the speed, the less likely an atom will be traveling that fast, and the Gamow peak is the resulting "most likely" speed at which to see fusion take place.
The Gamow peak is a factor in nucleosynthesis, such as the fusion taking place in stars.