Gasoline is one of many products produced from crude (others include kerosene, diesel, heating oil, butane, propane, plastics, etc...). You can also get a different mix of products depending on the grade of oil and refining technique, so gasoline has its own supply/demand dynamics which partially intersects with demand curves for the other products while crude is merely the input.

So cheap crude does allow more room to lower the price of gasoline and cheap crude is often linked to lower demand, however they're not in lockstep due to the many other factors.