It depends. Forza has basically split in two, with the Forza Horizon and Forza Motorsport series. Horizon is more of an arcade game, and Motorsport is aimed at more simulation fans like Gran Turismo.

Gran Turismo's direct competition, Forxa Motorsport, is actually pretty dead, and Gran Turismo is much more relevant there. But Forza Horizon is pretty much the most popular of all racing games right now, as an arcade game will always be more popular than a more simulation focused game like Gran Turismo. (Even if GT is not the most accurate)