Yes gars are illegal in cali. And I highly doubt its a spotted gar. What they are selling is 99.9% florida gar for $15. It has spots so they call it a spotted gar.
...But to comment on your title question, I've seen some pretty big true spotted gar in the wild. I seined one last weekend that was right at 46 inches long, but that's the longest one I've ever seen. 36 to 40 inches is not terribly rare in east Texas.