Can someone explain what this? Is it a fork of Android with the Google service apps rewritten? Or just the latter?
Could I take an old Android OS and install the alternative Gapps?
Yours, confused.
Only difference when comparing against LineageOS is that the OpenGApps package is "free".
Rather GApps (google) functionality is re-implemented from scratch and the necessary means (app signature spoofing) to replace GApps with microG is built into this LOS fork
I don't know for sure but probably not, the services aren't UserReplaceable.