AOSP established ROMs not built with Google services pre-installed, you have to manually install them by flashing a compatible Gapps bundle from TWRP or some other type of custom recovery. If you're planning to improve your phone or wish to flash AOSP 9.0 established ROM, then you have download Gapps to get Android Pie.
The majority of the programmers are using Linage OS as the foundation for their ROMs, and many of other customized ROMs like Resurrection Remix, ParanoidAndroid, OmniROM, SlimROM, etc. are all based on it. So you require Android Oreo Gapps 9.0 package for all these ROMs.
Types of GAPPS
Package Size : 1GB
This is a graphical installer which gives you the option to select which Google apps you want to install when you flash the file in recovery.
Package Size : 490MB
Stock is the most appropriate package for most devices, comes with all Google Apps that come pre-installed on Google’s Pixel and Nexus series phones.
Package Size : 441MB
This is a slightly modified Stock Gapps package which doesn’t replaces some of the AOSP apps like Browser, Camera, SMS app, etc
Package Size : 228MB
Mini GAPPS is the ideal package if you don’t want to go in with all the Apps Google offers on newer Android devices. It contains the popular Google apps, plus some more apps that aren’t up for download on the Play Store.
Package Size : 142MB
This is even more tight than the Mini package, contains only the most popular apps: Gmail, Calendar and Google Search (Now), plus some more apps that aren’t up for download on the Play Store.
Package Size : 91MB
Nano is another level down from the Micro package above, but it still includes extra apps that aren’t up for download on the Play Store.
Package Size : 49MB
This only gets you the the Play Store app, plus framework files required to make Play Store work. Nothing else.
How to Install Android Oreo GAPPS 9.0
- Download the appropriate Gapps package according to your phone configuration (see the below image)
- After downloading the appropriate Gapps for Oreo package Copy it in your in you android device
- Now reboot your device into recovery mode.
- Now select Install Zip from SD card option.
- Flash the gapps file in you android phone.
After flashing Restart you device and it will take time in first start up so be patient. You have now successfully installed all Google Apps on your android Pie 9.0 device.