If you have a membrane under the gravel self-scattered seeds will be unable to grow, even if they germinate in the gravel, as they will be unable to put roots down into the soil.
So, if you want your poppies to spread you should gather the seeds as the pods ripen, and sow the seeds in a seed tray and then pot on until you have sturdy plants. Then scrape back the gravel and cut a cross in the membrane where you want the plants to grow, fold the corners under and plant them in the soil (adding some soil improver/compost etc ).