Google Apps or Gapps are set of play store services and Google apps such as Google Maps, Gmail, Google contacts and other published by Google.
When you flash a custom ROM on your mobile, you need a Google Apps package according to your mobile’s CPU architecture and Android version.
Below are all the links of Google Apps or Gapps for Custom Android Oreo Rom’s such as Lineage OS 15. There is 7 type of packages for each architecture, the base version is PICO which is lightweight in size and doesn’t include many Google apps and lacks the “Ok Google” feature. While the other model has an increasing number of Google Apps.
Download OpenGapps for Android Oreo (Android 8.0)