OpenGapps offers several variants. Here is the breakdown for Android 5.1:
This method ensures no conflicts.
To restore your access to the Android app ecosystem on a legacy 5.1 device, you must manually select, download, and flash a compatible GApps package. 1. Understanding GApps Components for Android 5.1 gapps android 5.1
If you try to install a "Full" or "Stock" GApps package on an older device, you will likely get a "System Partition Full" error during installation. To combat this, developers created different "flavors" of GApps: OpenGapps offers several variants
If you already booted into Android 5.1 without Gapps: you must manually select
In TWRP, wipe Data (not Internal Storage). This resets the setup wizard. You will lose app data but not media files. Then reboot.