How can a gene drive spread if it causes a decrease in the fitness of the organism that carries it?

Decreased fitness (relative competitiveness) results in fewer offspring contributing to the next generation, but if all or most of those offspring possess the gene drive then the gene drive can still spread. As long as the inheritance advantage gained from the gene drive is greater than any associated fitness disadvantage it might cause, the gene drive is predicted to continue to spread and increase in prevalence.