RESTAURANTS : LEAN AND MEAN : Fat-Free Pizzas, Cheesecake, Tacos: Guilt-Free Gorging! Too Good to Be True? Maybe.

Take a number,” prompts the sign. We all crowd in front of the well-filled deli case, zeroing in on the salads, the triple-layer chocolate cake, the muffins. With its bright yellow walls, asymetrical chairs, a few tables out front and a mob of vocal regulars, Gratis has the casual ambience of a cafe.

The cash register spits out receipts in a steady rush. Number 37! I give my order: grilled vegetable salad, herbed ricotta and dried tomato pizza, soft tacos, spinach-and-mushroom lasagna, vanilla-bean cheesecake and a piece of the special peach cobbler.

Waiting, we eye the food some more. “Everything is fabulous,” gushes a seriously slender woman in faux pearls and Chanel. “And the potato salad is to die for!” Two large women confer heatedly over the menu. “No, no, let’s get the bow-tie pasta and the grilled Japanese eggplant and shiitake mushroom pizza,” says one. “And the herbed ricotta ravioli. And the vegetarian chili--that’s really good.” A newcomer to this 4-month-old Brentwood cafeteria looks over the entirely fat-free menu and bursts out laughing. “How can they possibly make a fat-free cheesecake that tastes any good?” he wonders, looking pointedly across the street at the Cheesecake Factory.

For lots of people (I am not one of them), all this fat-free food is too good to be true. Pastas, pizzas, chocolate cake. You can have it all. If you’re a dedicated fitness buff, on a strict no- or low-fat diet or simply watching your weight, Gratis is not only fat-free but also guilt-free.

The food is also pretty much taste-free. Granted, the cooks are working with a stiff handicap: no oils, not to mention butter, pancetta, bacon or even nuts or olives or corn, all of which have fat. No fish or meat or poultry either (even the leanest is not lean enough). Many of us could stand to reduce fat in our diets, but to banish it entirely is tough. It soon becomes clear that it is a very big leap from low-fat to fat-free--and one few of us would willingly take if the consequences were not so dire.

General manager Dale Greenblatt, who cooked at Spago and Spago Las Vegas before joining Gratis, spent a year and a half developing the recipes with the help of a consulting food technologist. He has some good ideas, but he needs to take the flavors further. At this point, breads and desserts are the most successful items.

There’s nothing deprived about the straightforward flat bread with a garlicky garbanzo spread instead of butter. Blackened from the oven, the pizza dough has a good texture and chewiness. Nonfat mozzarella melts like the classic, and herbs deflect attention from the ricotta’s chalkiness. Add a topping of sun-dried tomatoes or shiitake and eggplant, and this pizza is a reasonable replica.

Greenblatt has figured out how to mimic the textures of chocolate cake and cheesecake using no butter and “fat-free cream.” Not bad. Not bad at all. If you’re busy talking, drinking coffee or distracted in any way, you just might be fooled--for a couple of bites. Dark and bittersweet, the triple-layered chocolate cake does have a moist crumb-like texture, but the taste doesn’t linger in the mouth. I’d rather have one satisfying bite of a real cake than a huge slice of this version. The illusion is more credible in the dense vanilla-bean cheesecake topped with fat-free sour cream. The decent creme caramel has real caramelized sugar (only fat, not sugar, is banned here.) But the muffins, the poor, excruciatingly sweet muffins, have the texture of plastic foam.

The best of the soups is a gently spiced lentil. Black beans one day were grossly undercooked, and a harsh, acidic gazpacho tasted like doctored tomato juice. The unappetizing-looking salads are mostly a sorry lot. Soggy grilled vegetables have a gray cast and are doused with sweetish balsamic vinaigrette. Lentil salad tossed with a confetti of bright peppers is OK. The touted potato salad looks right, but in the mouth, the “mayonnaise” is just moist stuff with zero taste. Tabbouleh is in desperate need of handfuls of parsley, cilantro and scallions--no fat there, so why be fainthearted? Tasti

est of the lot is the red and green coleslaw in a “creamy” dressing and the substantial bow-tie pasta salad with sun-dried tomatoes.

When it comes to pasta, there is no reason you can’t make a fabulous fat-free marinara sauce with deep-flavored tomatoes and fresh basil, but Gratis’s metallic-tasting marinara does nothing for the huge slab of bland mushroom-and-spinach lasagna or the leaden ricotta-filled ravioli. The soft taco, an organic-wheat tortilla filled with shredded lettuce, a sweet, orange-brown vegetarian chili and squiggles of fat-free cheddar, is truly awful.

If I ever have to go fat-free (and I pray that I don’t), I’ll be dreaming of a fuzzy ripe peach dripping with juice or an unadorned vine-ripened tomato at the height of the season. I will not be dreaming of Gratis.

Gratis, 11658 San Vicente Blvd., Los Angeles; (310) 571-2345. Beer and wine. Validated parking in rear. Dinner for two, food only, $18-$39 .