Most of the developers already got working Android 9.0 Pie (AOSP or Lineage OS 16) build on their devices. AOSP based ROMs don’t come with Google services pre-installed, so you'll need to download and install Android Pie based Gapps on your device otherwise, you won’t get Google Play Services on Oreo based ROMs.
For now only Micro Gapps package is available for ARM64 bit processor. Before downloading you'll need to find your device's CPU type. You can install CPU Z that gives you a readout about your device's hardware.
Download Android 9.0 Pie Gapps
Open Gapps (Micro) for ARM 64
How to Install Android 9.0 Pie Gapps
- Download Gapps from the above link
- Flash your Custom ROM (LineageOS 15 or AOSP)
- Install Gapps