Honestly, I think it depends on where the car's original design hails from.

While the majority of GM cars on the left, those with European and Asian influence tend to be on the right. E.g., Pontiac G6/Chevy Malibu (Epsilon platform, which I think started with the Opels) and the Chevy Aveo (from Daewoo).

Those with a purebred American design have the gas caps on the left. E.g., The Chevy Impala and the newer Toyotas (designed for the US market in mind, and designs sent back, which is opposite of what it was before).

I can't explain the Taurus... it's just a ....... car anyways

The historical reason is that while American gas stations were drive in, those in Europe and other foreign lands had curbside gas stations, which necessitated the gascap to be on the right side.