Explanation:
If you are not a gerbil, then you are not a pet owner. False; if you are not a gerbil, you can be a human who owns pets.
If you are not a pet owner, then you have a gerbil. False; if you are not a pet owner then you have no pets.
If you do not have a gerbil, then you are not a pet owner. False; you could have a dog or a cat.
If you are not a pet owner, then you do not have a gerbil. True, since if you do not own any pets, you will not have any animals, so you will not have a gerbil.
Hope this helps!