"I have a different risk assessment than my colleagues participating in the Opel negotiations," Guttenberg said in a statement. "Certainly an insolvency would also not currently be without risks."
As part of the deal, all four factories in Germany would stay open though Magna, based in Aurora, Ontario, has said previously it would need to shed some 2,600 jobs.
Opel employs 25,000 people in Germany, nearly half of GM Europe's work force.
German government officials said Magna's plan anticipated between 7,500 and 8,500 job cuts across Europe.
Opel and Vauxhall have operations in Belgium, Spain and Poland among other countries.
Merkel said Germany wanted to work closely with other European countries with GM Europe facilities as the plan developed.
"We are in close contact with Opel's European partners and the European division of GM," Merkel said.
AP Auto Writer Tom Krisher in Detroit contributed to this report.