A resistor also has potential difference. See, a resistor differs from a battery in that current flows from - to + but for the resistor it's + to -. EMF makes the difference.Obviously a potential difference can be defined because that is how we normally define a battery.
A battery is defined by its emf, not its potential difference. A resistor has no emf across it.
How many orders of magnitude is gravitational force typically less than electric? You'd have a hell of a time trying to move an electron with gravity ...........gravity...................
So show me a massless charged particle