I think the gerbil myth arose because wild caught ball pythons would often refuse to eat domesticated rats and mice. They were too different from what the animals were used to. A gerbil smells distinctively different from a rat or mouse--so does an ASF. They would sometimes tempt these non-feeding wild-caughts.
Ball pythons can be stubborn about their prey, in captivity. However, a ball python that has been eating rats or mice will continue to eat rats or mice--I guarantee it. They know that it is a food item. If they accept an ASF or gerbil, they were probably getting ready to eat their normal prey again anyhow.
I have had many of my snakes refuse ASFs when they were offered to them...because they wanted a rat, which is what they were used to eating. I've seen no sign that ball pythons consider these other rodents to be more appealing than rats or mice.
I have seen signs that newly hatched ball pythons prefer mice over rats for their first meals, but I've also had a few refuse mice, and then take rats. I had some refuse both and take an ASF. I've had none that did not eventually switch to rats or mice, and only a very few ardent mousers. (Also a few ratters, that would not take a mouse, ever).
I strongly believe that the most appealing food for your ball python is the species of rodent that it is MOST accustomed to eating.