A mechanic is someone who repairs vehicles. If your car breaks down, a mechanic would fix it for you. In the past, there would be a 'pump attendant' who put the fuel into your car. But nowadays, pumps are all self-service, so the only people who work at a gas stations/petrol stations are the cashiers who take payments.

What you call them will change depending on where you are, and what position they fill. In Eastern United States, I hear the man who stands by the pumps an "attendant" and the man who stands behind the counter within the little gas mart a "clerk" or "cashier". I would not, nor have I heard anyone call them a mechanic. Hope this helped :)

If he fixes your car he is a mechanic. If he sells things at a gas station, he is a clerk