Most new substratesd will not have a lowering effect on the tank water, as they age, they get sour and more organic matter builds up, the older gravel likely did this, whereas this new gravel, will take some time, the EC sometimes will raise it.
Flourite will not impact pH at all, I've never heard anyone ever say anything about flourite doing anything to the pH/GH/KH.

Regards,
Tom Barr