It's fine.
A lot of people would say '...it had gone'.
Is this sentence OK? Is there a better way to say it?
She forgot to retrieve her luggage from the rental car and when she went back to look for it, it was gone.
It's fine.
A lot of people would say '...it had gone'.
Regardless of "had" or "was", the difficulty remains that, as written, it's not clear whether her luggage had gone - or whether the car itself had gone [with her luggage in it!].
Better to say "She forgot to retrieve her luggage from the rental car, and when she went back to look for it, found that her bags [or the car] had gone".