"We can't say what could be expected from a President Donald Trump," Gauck told Germany's Der Spiegel weekly. "To me, and to many people in the United States and here at home, this constitutes a problem."
With just two days to go in a closely fought election that is being watched around the world, Gauck added: "When I look at Washington, I am worried."
The German president, whose role is largely ceremonial, said he hoped "that American democracy, which does not allow the president to act as an autocrat, that the system of checks and balances, of mutual control" of power would be maintained.
The latest major survey, the ABC/Washington Post tracker released early Sunday, gave Democratic candidate Hillary Clinton a five- percentage point 48-43 lead over Trump. Polling averages however are closer.