MindTheGapps (are) unofficial Gapps for Android 8.1, as Official Gapps is available for only Android 8.0. With LineageOS started working on version 15.1 and rolled the update for some device including Oneplus 5T and Google Nexus 6P. There is also unofficial Lineage OS for many devices. If you are flashing a Lineage OS 15.1 or other Custom ROM based on Android 8.1, download and flash the Gapps for 8.1 packages.
What is MindtheGapps?
Mindthegapps is an unofficial version to support custom ROMs based on Android 8.1. You should use it until the arrival of the official Gapps that support Android 8.1.
Just like Gapps, MindtheGapps enables Google play store functionality on your Android device. And you should flash it along with custom ROM. If you don’t use it, you won’t be able to use Google apps like Youtube etc, sync contacts and data or download apps and games from the google play store.
Gapps 8.1 ARM
If your device runs on ARM CPU, you’d need ARM version.
Gapps 8.1 ARM64
When your device runs on ARM64 CPU, eg. Oneplus 5t, Redmi Note series.
Gapps 8.1 X86
It’s unlikely that you don’t need Google play store and play store functionality, for the reason Gapps is there. Follow the link and download a suitable version and flash it on your computer.
How to flash Gapps 8.1 on Android
First, you need a custom recovery like TWRP or other. You’ll understand this as you already using a custom ROM (or going to flash). The process is simple, you unlock your phone’s bootloader (using the fastboot oem unlock).
Then you need to flash a custom recovery using fastboot boot ###.img (replace ### with recovery file name).
When recovery opens, select Install > Select the Gapps package > slide to flash it. Restart your phone and Google play store will be there.
If you are flashing Gapps after you installed custom ROM, you can try clearing Cache to avoid app stopped working message.