You need TWRP (Team Win Recovery Project) or OrangeFox Recovery. Stock recovery will reject unsigned GApps ZIPs.
Follow these instructions precisely. Assume you have already flashed a custom Android 6.0.1 ROM (like LineageOS 13.0). gapps android 6.0.1
This feature allowed users to format external SD cards as internal storage. While great for users, it complicated GApps installations. If you plan to use Adoptable Storage, you generally need to install the GApps package before encrypting or formatting the SD card, or risk the link between the app and the storage breaking. You need TWRP (Team Win Recovery Project) or