OK READY,

I almost hate to, oh well.

You can purchase inflatable snakes, YES, snakes. (large ones, 4" in diameter by at least 6')
Cut the tips of the tails off and stuff them full of something, I havnt used this but I think foam insulation in the cans you purchase @ Lowes or Home Depot would be better than cotton or rags, itll be waterproof.
Youll need to anchor these decoys to the asphault, as far as how many, itl be trial and error.
You will have introduced a very natural enemy to the gulls, thell look for a more inviting dining table untill the other guests leave, of coarse, they never will.
Go ahead WOOOHAAWWW away, it works!
My uncle had birds for years nesting in a garage, with no doors and making a mess out of everything, he got those snakes from some gardening book, ordered them, stuffed them put 2 in the garage and I SWEAR, the birds abandoned their newborn and NEVER returned.

Try it!

Roger