I need to reinstall (OS? and gapps) after a part-wipe of internal memory of Manta, 5.1
In order to avoid a bricking, what is the safest option?

You're not going to brick the device by installing stock or cyanogenmod.

Should/Can I transfer the Android stock to the nexus and install that?

This is the safest option but will wipe all your data. You don't need to "un tgz" it. This will take you back to 5.1.1 that was updated earlier this year
See https://developers.google.com/android/nexus/images for instructions

Be brave and install the Cyanogenmod 13 and Gapps?

If you are going for a 6 custom version of android also look at the early AOSP builds or OmniROM. I'd recommend a factory reset in recovery, flash SuperSu  (if you want root) &amp; GApps then flash the ROM. OmniROM have started doing v7 builds

CyanogenMod deals with SU for you.