Clock speed of Adreno 509
According to Wikipedia, the "stock" clock for Adreno 509 (Snapdragon 636 SoC) is 720 MHz.

But keep in mind that this "stock clock frequency" on Wikipedia is the "stock" defined by Qualcomm, and it's extremely common that phone manufacturers define a different (higher or lower) "stock" clock for chips.

For Moto Z3 Play, the "Lenovo stock" frequency is 850 MHz, as can be seen on their website.

I have not been able to find a "max speed" of this particular GPU, probably because my Google-fu isn't powerful enough.