Can't say I have ever tackled seagulls poop but I can only imagine how tough it is once dried on. I would think that making a paste of something like dawn dish soap or mabey simple green and let it soak. Worse case is to get something like an oven cleaner but monitor and test it on a non noticeable area first. They make non marking scouring pads to remove crud and grime with no damage to the surface......good luck...