Gorbachev. using the powers he gained when the Congress of Peoples Deputies elected him the Soviet Unions first executive president last week. said Lithuanians had a week to turn in their firearms. If they fail to do so. he said the Soviet Interior Ministry should be prepared to confiscate them. Gorbachev also told KGB border troops to tighten their watch over the republic and told officials to reduce the number of visas issued to foreigners seeking to visit the republic. His decree was the latest in a series of increasingly tough moves taken by Moscow in an attempt to discourage secession by the republic of 3.8 million people. It came less than 24 hours after a strong protest from Lithuanias government over stepped up maneuvers by the Soviet military in the state.