In the GOM, nothing works that I've ever seen. I've seen systems that made noise of various frequencies, I've seen fake owls, eagles, snakes, and just about anything else you can think of, and none of them did any good. Apparently they aren't afraid of any predators, because they tend to sit close to the fake owls and eagles. Painting pictures on the deck does no good because it just gets covered with the seagull s**t. I've landed on decks that had well over an inch of it, along with the bones, fish scales and feathers that always are there, and when I finally refused to go there again, the company got some high-pressure water blasting machines to clean them. They had to remove paint, because the stuff had penetrated well into the paint, so a repaint was needed. After all this, the gulls just came back. For some reason, they like certain spots, and will congegrate there no matter what the humans do, and won't even bother other platforms close by.
About all I can suggest is a high-pressure wash every so often, the frequency depending on the rate of buildup.