If you're calling about a franchise, forget it,'' warns a counterman at Gelati per Tutti, an Italian ice cream shop on voguish Melrose Avenue. ''People are calling for franchises left and right.''At the headquarters of Gelato Classico in San Francisco, franchise inquiries come in at a rate of more than 10 a day, according to John Heffernan, who owns the company.Gelato is what Italians call ice cream, and many say Italians have always made the world's best ices, from the days when Nero sent runners into the Alps to fetch him bowls of snow.Now Italian recipes for gelato have become an entrepreneur's dream, winning over American palates by the spatula-full. Gelato (meaning ''frozen'') is soft, rich ice cream with very little air in it, and flavors as clear and fresh as Lake Como on a summer day.When Brooke Shields comes to town, she reportedly stays within walking distance of the Gelato Classico shop in Beverly Hills so she can cool her palate at whim. ''She comes here three times a day,'' avows store manager Avi Mishal.Celebrities come here daily, Mr. Mishal says. One double-parks his Porsche in front of the store, gets a ticket from the ever-vigilant Beverly Hills police, and says it's all worth it.''It's chic ice cream,'' says Mishal, a former Israeli Army officer, with a shrug. ''It's the most affordable luxury in Beverly Hills,'' he says, quoting a newspaper review. People from all walks of life come here, he adds.The gelato juggernaut began to roll in San Francisco about five years ago, when lines at shops like Gelato Classico grew long and started spilling onto the street.Now gelato shops are bursting out all over that great market of markets, southern California, and they're beginning to spread east. Gelato makers are at that euphoric time for a young industry when the public wants more than the producers can provide.''It's snob appeal,'' says Leonardo Sudman, president of Tartufo Italgelateria, the nation's largest gelato maker. ''But people who can't afford a Ferrari can afford $2.49 for a pint of ice cream.''Mr. Sudman runs what looks like a storefront operation in San Gabriel, a town near Pasadena on the eastern side of the Los Angeles metropolis. It could be a freight shipper's office.But stretching out behind the front offices are the big, Italian-made gelato freezers. White-robed workers grind up fresh cantaloupe, and cold-storage rooms play frosty olfactory symphonies from notes of chocolates, oranges, melons, and hazelnut.Proud as a new father, he has a few cups of gelati brought into his office. The canteloupe flavor is just out of the freezer it was made in. A visitor savors a spoonful and stumbles after the words. ''It's just like. . . .'''' . . . like eating a fresh cantaloupe,'' Sudman finishes, smiling contentedly.He's right.As the nation's largest gelato maker, Sudman's firm is really not very big - not yet - but Sudman is aiming high. New machines now being hooked up in his back rooms will expand capacity four or five times over. By the end of the year, he says, he'll be selling gelati in 35 to 40 states, if not in all 50.Unlike most gelato makers, who sell to little ice cream boutiques, Tartufo Italgelateria supplies restaurants and grocers, including some big chains like Ralph's and Safeway.This year, sales will amount to some $5 million. Within five years, the robust Sudman hopes for yearly revenues of $50 million. ''It could be $500 million.'' He raises his eyebrows happily.The company started without any such grand ambitions. Sudman had spent 27 years as an importer of Italian goods when a friend with an elegant Italian restaurant found the only way he could put good gelati on his dessert menus was to make it himself.Sudman became a partner in the operation. Soon, other restaurateurs were buying gelati from the plant, and diners wanted to buy pints to take home. Tartufo Italgelateria began to grow.Sudman gives much of the credit to Haagen-Dazs, the New Jersey-born star of superpremium ice creams. ''Haagen-Dazs broke the dollar-a-scoop barrier for ice cream,'' Sudman says. The rest of the ice cream elite has followed in its trail.Most Americans have yet to know the joy of gelati. Even in up-to-the-minute Beverly Hills, most people come to Gelato Classico cold, so to speak, having only heard of, never tasted, the Italian delicacy.While there have always been little mom-and-pop shops selling Italian ice cream in this country, only in the past few years has the fashion really begun to spread.When John Heffernan, after 20 years of corporate life, bought Gelato Classico in 1979, the company consisted of two shops in San Francisco and a recipe for ice cream the previous owner had put together during his travels in Italy.Now there are eight Gelato Classico shops in the San Francisco area, one in Beverly Hills, one in Scottsdale, Ariz., two recently opened ones in Michigan , and one just appearing in Westwood Village in Los Angeles.''I don't think we've scratched the surface,'' says the cautious, methodically spoken Mr. Heffernan. ''We are going nationwide as the right opportunities present themselves.''One of the early gelato peddlers in southern California, San Francisco interior designer Darlene Welch, was in Los Angeles for a conference a couple of years ago, went out for a Chinese dinner, and was stunned to find there was nowhere to go for gelati.She fixed that with her year-old Gelati per Tutti on Melrose Avenue, designed as an authentically stark Italian marble-and-plaster shop. Gelati per Tutti gets its gelati from Vivoli's in Berkeley. The product, too, is authentically Italian, says Ms. Welch.''We don't make milk shakes.''Now, one of the big guns of the dairy business is putting one foot forward into the frozen fray. Borden Inc. has begun making some 50,000 gallons of ''Gelare'' - a recipe it bought from a five-shop chain based in San Luis Obispo, Calif. - for five months of test marketing at supermarkets in Houston; Dallas; Pittsburgh; and Columbus, Ohio.Like every young booming industry, gelati is drawing hordes of newcomers into the business. John Heffernan, deluged with franchise requests, sees a shakeout in the gelati industry's future.''People see the lines coming out of our stores and want in the business,'' he says. Many, he adds, are ill-prepared. ''A lot of people will go out of business.''It's an expensive business to get into. Each of Leonardo Sudman's three new freezers from Italy costs $50,000. He uses only fresh, seasonal fruits. And preparing each batch of gelato is a 12- to 14-hour process. Sudman's gelati are high in butterfat, 19 percent, and low on air, 9 percent, compared with 50 percent for many American ice creams.The Italians have been the standard-bearers for ice cream throughout the centuries. The common lore is that Marco Polo brought back recipes for water ices (sherbets) from the Orient in the late 13th century. They caught on with the Venetians and spread. Catherine de' Medici carried the fashion to France in 1533 when she married Henry II.No one knows who began using frozen milk and cream instead of water, but it may have already been the practice when another Italian named Coltelli opened the Cafe Procope in Paris in 1660, which helped popularize frozen desserts. By the early 18th century, English travelers were writing home about the ice cream on the Continent.Now stores in Hanover, N.H., Brooklyn, N.Y., and Carefree, Ariz., are writing Leonardo Sudman about getting deliveries of his crema de la crema Italian ice cream.