You did the right thing then. I had to return a gerbil for the same reason about a month ago, but this gerbil I was slowly trying to introduce to my other gerbil. I felt bad returning him too, but I bought him for companionship for my other gerbil, and I wasn't about to go out and buy a new tank and accessories for a gerbil that attacked my other gerbil.
Maybe later down the road you can introduce another gerbil to Dipstick. If I were you (if possible) I would find a gerbil breeder, so the breeder would know the tempermant of a gerbil that you are looking for and find the perfect match for Dipstick. That is the mistake I made, I tried introducing a new gerbil that I didn't even really know his tempermant yet.
How old are these gerbils at the petstore? I wonder if they put all sorts of gerbils together at the petstore from different shipments and then give them all something so they won't fight in the customers prescense. Some petstore I have seen, I wouldn't doubt it.