Alvaro Gonzales contract with Marseille runs till 2024 so its unlikely that they would just let go of the player on the cheap especially as he's featured six times for them and is in their existing plans. I've read that he has some sort of clause that can have him leave this winter, I'm not too sure if that is correct, but I'm reporting it anyways because it might be correct and that might be how he joins Valencia CF.
As far as I'm aware Valencia CF want to loan the player for 6 months with an obligatory purchase clause at the end of the loan deal.