Gapps must be flash'd directly after ROM, before first system boot. Smallest package, Pico, is recommended. SU can be flash'd at that time or later, but... You need to use the correct SU add-on, call'd addonsu-16.0-arm-signed.zip, available here:... wiped and reinstalled ROM, gapps and then SuperSUv2.79-201612051815 ...
https://download.lineageos.org/extras