Depends on the flow. Make a estimate for the return storm (25 or 100) based on the local regulations. If it is just gasoline , go with the gravity separtor based on API design Pub 421. It is much cheaper and with almost no mechanical components.
mechanical systems are costly and tough to maintain too.
Gasoline will evaporate very quickly into the air if present. Any enclosed structure will quickly become a very hazardous area.
Also, an oil/water separator is not going to be very efficient at removing low concentrations of gasoline.
And it is not likely that significant concentrations are ever going to be present. I have to believe that it is very, very unlikely to have any significant concentration of gasoline in storm water since it will be an extremely hazardous event to have gasoline present.