I am not specialist on this but i would describe it as this:
example:
Maria came to see me, but I was gone.
I believe that
"I was gone" is passive voice of simple past tense
see this page:
Learn English - Passive Voice
we can look at it as association of status/condition, so my status when Maria came to see me, was "gone", i know it is a verb as word, but look at it as my condition, so rather as adjective associated with me.
example:
Maria came to see me, but I had gone.
this is past perfect tense
so this time look at it as on full verb construct consisting of inseparable components of "had" and "gone", which together makes a single verb of specific tense. And this verb expresses my activity in past... which is i have got to other place even before when Maria came to see me.
what is better to use? I believe (although i might be wrong), both ways have same meaning in result, but first is focused on expressing someone's status which would anyway be a result of such activity which expresses a second example
so the second one is focused on describing of activity which happened before the first part of sentence (or the context generally).