Success is no secret, if one can learn from Le Gaulois. It is simply a matter of very good food at very good prices. It doesn't matter that the place is small, the tables are close, the stairs are narrow, the decor is Holiday Inn, the service is rushed. The food is, was, remains, and hopefully always will be, wonderful. And those wonderful meals often cost less than $10 a person when all is said and done and eaten and tipped. Besides the few continuing salads, sandwiches, quiches, crepes and "cuisine minceur" dishes, the heart of the menu is the daily specials. Start anywhere, it is hard to miss. Even fresh fruit salad is a sprightly combination with - at $1.50 yet - fresh raspberries in the middle of winter. Beef tongue is a rare find in Washington; here it is tangy with vinegar and mustard, spiced with shallots and parsley, and truly grand. Scallops are cooked just to a tender, juicy state. Fish dishes are repeatedly excellent. Crepes are delicate, richly filled. As for desserts, the pastries are home made and well made, the puff pastries crisp and buttery, the creams rich. Tia Maria mousse is soft as a pillow. It is the kind of restaurant where people leaving stop people coming to tell them what to order bcause it was so good.