I´ve tied OP3 charger. It´s charging but won´t give you fast charging. Charging current is only about 1 400 mA compared to 3 300 mA of the k20 charger and other quick charge compatible chargers.
ionflux posted a guide how to make G-pay work.
Just follow last paragraph in this post: https://forum.xda-developers.com/showpost.php?p=79859363&postcount=1
Worked for me on xiaomi.eu.
Dirty flashed over the last build and works flawlessly.