I would have to say that Gran Turismo is still a more popular franchise and Sony still has more racing fans but the comparison is skewed by the fact that the output of Gran Turismo games this generation has been weak given that we may not see another full version released in this generation.
In the next generation expectation ought to even things up a little between the franchises as I doubt that anyone expected Forza to be as comparable to GT in this generation when they purchased their systems. GT looks relatively weaker because there hasn't been the opportunity for people to buy iterations of the franchise. The relative sales of these franchises are probably predicted base don the relative sales of the Xbox Next and PS4, if the PS4 sells 2:1 over the Xbox Next then I would expect the relative sales of GT to be >1.5:1 and vice versa. You do have to account for hardcore gamers who will own both systems are are attracted to the 'best'.