Do I have a case after I fell on the premise ground when I was heading to the cashier to pay gas because of the gas that has been spilled on the floor

In Florida, merely because someone is injured on the property of another does not make the property owner responsible. You would have the burden of proving that the gasoline had been on the ground for a period of time that the owner, in the exercise of due care, should have known of its existence and failed to clean it up. I think that would be very difficult to prove after the fact. Typically, gasoline is not a slippery substance, and there was probably oil that had soaked into the concrete and the spilled gas only brought it to the surface. In any event, this doesn't sound like a provable case to me, but that's just my opinion.