As I understand as per literature reading and some khanacademy(https://www.youtube.com/watch?v=PtKAeihnbv0&t=315s):
Proton gradient := the gradient created when hydrogen ions moves to the intermembrane space from the mitochondrial matrix.
Mitochondrial membrane potential := the potential energy created by the proton gradient.
Does depolarization mean the loss of mitochondrial membrane potential? The lower the mitochondrial membrane potential, then the more depolarized the mitochondria?
So the way I defined proton gradient and mitochondrial potential is conceptually derived from the so-called proton-motive force used here https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2605959/ and also described here https://www.dummies.com/education/science/biology/the-proton-motive-force/ as an electrochemical force. Thus, my definition is from my rudimentary physico-chemical intuition.