Open GApps is a project that includes all official Google applications to be installed on custom firmware. Here is the package compatible with Android 9 Pie
Where to download the GApps compatible with Android 9 Pie
Whoever installs ROM, custom firmware and modding face a smartphone usage philosophy will surely have downloaded and installed some ROM, see the LineageOS, which have not included all the Google Applications the so-called GApps in the ROM that go flashate apart with a second file .ZIP always from recovery.
Other examples are the Paranoid ROMs and all AOSP ROMs that do not include any of the GApps, including the Play Store.
Without getting lost in the wrong downloads on the basis of our ROM comes to help the usual community of XDA (user Nezorflame) that through its developer has already created the GApps to be installed on custom firmware based on Android 9 Pie.
Open GApps is a completely open source project that deals with indexing and making available all Google Apps for each ROM so as not to install the wrong ones.
At the moment the GApps are not official as not yet released by Google, but all modders can integrate them into their custom firmware and have them installed to users as they are not integrated due to legal problems with Google.