Biologic drugmaker Gilead Sciences Inc. said Monday that it will buy a subsidiary of Nimbus Therapeutics LLC and its experimental pill for an increasingly common metabolic disorder that causes life-threatening fat buildup in the liver. Gilead, based in Foster City, California, will pay $400 million for Nimbus Apollo Inc. Parent company Nimbus Therapeutics, based in Cambridge, Massachusetts, could receive another $800 million if Nimbus Apollo's drug development program meets certain milestones in testing results and medicine approval and sales.