Google Apps a.k.a Gapps Pack is must have on android in order to run any google apps like Playstore, Gmail, google+ and other google applications.Most of the MIUI ROMs don’t come with the Google apps so users have to flash Gapps Zip files separately. Your Gapps package not only includes basic applications like Google Chrome, Hangouts, YouTube, and many more apps. There is a variety of Gapps packages. They range from as less as 50 MB to as huge as 400 MB. There are packages like Nano pack, Pico pack, Megapack, and full pack.
Download Gapps for MIUI 9
Download Google installer v2.0 apk
How to Install Google Play Store On MIUI 9
If you have any queries regarding this then please do ask in the comment section.