Ron probably has more details, but in my experience a "Gross Polluter" was a car that failed testing multiple times. So it might be that the "Check Engine" light is on, but for cars of older vintage, those that failed the emissions tailpipe testing.
I had two GPs back in the day when I lived in California - a '74 Toyota Landcruiser (which I eventually got to pass) and a '7? MGB (which I ended up selling at a loss because I was screwed, all the smog equipment had been removed, it had a weber carb, and it was failing).
The MGB I brought in from Florida before I understood how the CA smog process worked. Honestly, it was a nightmare.
Rocky