1. Place lamb on rack in shallow open roasting pan; insert meat thermometer into center of roast. Roast lamb in 325°F. oven until meat thermometer reaches 160°F. for medium (about 30 to 35 minutes per pound) or until of desired doneness.
2. About 30 minutes before roast is done, in small saucepan over medium heat, heat marmalade, sherry, and 1/2 teaspoon garlic salt, stirring constantly, until marmalade is melted; occasionally brush roast with mixture during last 30 minutes of roasting. Place roast on heated platter; remove strings.
3. In measuring cup, mix flour, 1/4 cup water, and 1/2 teaspoon garlic salt until smooth. Remove rack from roasting pan. Spoon off any fat from drippings. Add 2 cups water to drippings; stir to loosen browned bits. Stir in flour mixture. Over medium heat, cook, stirring constantly, until mixture thickens slightly. Serve gravy in gravy boat with meat.