A voltage gradient is a difference in electral potential over the cell membrane.
An action potential is something a little bit different, have a read of this site, I think you will find it helpful
Nerve Impulses
The action potential is the change in the voltage gradient, so cannot be used as an example.
If I were asked to give an example of a voltage gradient I would quote the K, Ca and Na concentrations and say outside of the cell is more positive. I would then say the mV that you can record inside of the resting cell (~-70mV) and outside* (using a voltage clamp to measure it- talking about experimental procedures always gives you extra marks).
*I've seen a few of these use whatever your lecture notes say.
Linear Mode
