The agency ruled Thursday that public comments gathered as part of an environmental assessment ordered by a federal judge did not warrant any change in plans.
The park service says the nonprofit Gettysburg Foundation has funds to carry out the demolition. Spokeswoman Katie Lawhon says work is expected to start this winter and to be mostly done by this summer's 150th anniversary of the Civil War battle.
The building once housed the Cyclorama, a 360-degree circular painting of a battle scene. The restored painting was moved to a new visitor's center in 2008.
The park service has planned to tear down the building since 1999.