Whenever we install any custom ROM, the one thing which is very important to keep in mind is flashing the Gapps package. Google last year released the latest version of its OS Android Oreo 8.1 along with the Android Open Source project source for Android Oreo 8.1. Developer with this source code can update their custom ROM with the latest versi0n of Android Oreo 8.1. Many famous custom ROMs developer like LineageOS, Resurrection, Crdroid, etc already created their custom ROM based on the latest Android Version.
So, if you are the one who flashed any of these custom ROM based on Android Oreo 8.1 / Pie 9.0 then you might be noticed that most of the custom ROMs won't come with Google Apps like Play Store, Gmail, Youtube, Hangout, etc. The reason behind it is that Android OS is AOSP but Google Apps are not, so you need to flash GApps package manually. The most important component of Gapps is Play Store because of it, we can download millions of Apps. So, if you flashed any custom ROMs based on Android 8.1 then you need to flash Gapps package as well in order to enjoy all the Google Play services on your custom ROM.
You can not flash any version of the Gapps, you need to do a little research before flashing Gapps as there are the different version of GApps available for the different android version. Here in this article, we have listed download link of Open Gapps for ARM and ARM64 Devices running on Android 8.1/9.0. Open Gapps are the most popular Gapps package, it gained popularity since its first release, It is the most recommended GApps package.
Open Gapps package for Android Oreo is still in Beta Quality. Now if you are using custom ROM based on Android 8.1 then you can flash Open Gapps package without any second thought, you don't need to use any alternatives of Open Gapps which aren't automatically updated.
Before downloading Gapps Package make sure which variant of Gapps you want to download as there are different Gapps package with different size are available at the download page of Open Gapps. With the increase in the size of the package, there is an increase in Google Apps. If your device internal memory is less then I would recommend you to go with the less size of Gapps, If you want only Google Play store and some basic Google Apps like Youtube, Gmail then downloads the smaller size GApps and if you want to get all the available Google Apps then download the biggest one.
Unofficial Open Gapps for Android 9.0 Pie
Here is our first unofficial version of Open Gapps for Android 9.0 Pie. Thanks to XDA Senior member: Nezorflame
Official Open Gapps for Android 9.0 Pie and Oreo 8.0 / 8.1
It is the most downloaded GApps package as it is very small in size, it consumes very few spaces in your internal storage and it has basic Google play service like Play Store, Youtube, etc and it does its job very perfectly. It doesn't have any useless Google apps which you don't use.
It is a bigger in size than the pico package and has some more Google Apps as compared to pico package.
It is bigger in size than Nano has some more Google Apps than Nano, like Youtube, Gmail, Hangout, Google Play Games, etc.
This package is made for those users who prefer to use only limited Google Apps. In this package, you will find Apps like Google Maps, Google Chrome, Gmail, Google Duo, Google Play Movies, Face detection for media, Face Unlock, etc.
If you prefer the option to use stock/AOSP apps, this package is designed for you. In this package you will find the core Google system base, Google Android Shared Services, off-line speech files, Face Detection for Media, Face Unlock, Google Play Store, Google Exchange Services (replaces stock/AOSP Exchange Services), Dialer Framework and the following Play Store applications:
This package is designed for those users who love all Google Application, therefore, in this package, you will find all the Google Apps which you get with any Google device.
Here is the direct link to download the Open Gapps 8.1 package for Android Oreo.
How To Install Open Gapps Android 8.1/9.0 Package
It's quite simple to flash the latest Open Gapps Android Oreo 8.1. After you flash the required custom ROM, simply flash the GApps provided by the ROM developer through the recovery. That's it.
So, that's pretty much about it. So, remember the points above and only flash the GAps that your custom ROM developer provided.