Have you got HDR switched ON in the PS settings menu?
Does your HDR work fine with other games?
There is something fundamentally wrong with GT7 HDR though, it looks really bright and washed out. When you try to address this in the settings the slider that you can change from zero to -/+ 1 does nothing. The Saturation slider next to it works though.
It's actually spoiling the game slightly for me, the HDR on GT Sport was so much better and Sport also had better HDR calibration settings
They really need to sort this out fast!