The generation gap is the fraction of the population to be replaced after each iteration (generation).  The value of (1 - generation gap) indicates how many individuals will be kept from the previous generation.  The generation gap ranges from 0 to 1.  Generation gap = 1 means that the

entire population is replaced by new offspring after crossover.  In the case of  elitist strategy, the value (1 - generation gap) determines the number of the best (elite) individuals passing to the next generation without the crossover and mutation operators.  The values for the generation gap are most commonly in the range 0.9 - 0.99.