For catsup, in a saucepan combine gooseberries, tomato paste, sugar, vinegar, finely chopped onion, Worcestershire sauce, and salt. Bring to boiling; reduce heat. Simmer, uncovered, 10 minutes, stirring often. If desired, press mixture through a fine-mesh sieve; discard skins. Cover and chill for 2 hours or up to 2 weeks.
Meanwhile, pierce holes in skins of bratwursts. In a saucepan combine bratwursts, beer, and peppercorns; bring to boiling. Reduce heat. Simmer, covered, 10 minutes or until thoroughly cooked; drain.
Grill bratwursts on rack of an uncovered grill directly over medium coals for 7 to 8 minutes or until skins are golden, turning often. Serve on buns with gooseberry catsup. Top with coarsely chopped onion, if desired. Makes 6 servings.