The German government has set out 14 conditions to decide whether to provide financial help for Opel's future owners, the Financial Times reported on Monday.
Conditions including setting up a headquarters in Germany, according to an internal paper by Frank-Walter Steinmeier, vice-chancellor and foreign minister.
According to the paper, all potential Opel buyers would have to say where they intended the company's headquarters to be located and where it would pay tax.
GM Europe is currently based in Zurich, Switzerland.
Other factors mentioned include the number of expected job losses and the future of the four Opel plants in Germany.
The bidder's past experience in conducting similar transactions, the solidity of the financial concepts and the buyer's degree of acceptance among Opel's workers will also be considered.
GM Europe has attracted interest from Italy's Fiat and a consortium led by Austrian-Canadian parts supplier Magna.
General Motors will decide which bidder will buy a stake in its European operations. GM CEO Fritz Henderson said he will continue talks with bidders in the next few weeks.
The German government will have an influence because GM has asked it to provide credit guarantees to help GM Europe overcome liquidity problems.
GM Europe is seeking 3.3 billion euros ($4.4 billion) in credit guarantees from European governments to help it overcome its liquidity shortage.
In Germany GM is seeking a credit guarantee of 2.6 billion euros.
Fiat CEO Sergio Marchionne was in Berlin on Monday to present his plan to merge Fiat's operations with GM Europe's businesses including Opel to German economics minister Karl-Theodor zu Guttenberg and to Steinmeier.
Magna gave a similar presentation last week.
The future of Opel has become part of the German electoral campaign ahead of a general election on September 27.
The contest will pitch Angela Merkel, chancellor and chairman of the Christian Democratic Union, against Steinmeier, who will represent the Social Democratic Party.
A GM Europe spokesman said on Monday that the company is in talks with several possible investors, but would not comment on their identity or the timing of negotiations.
"We see good and realistic interest of some of these possible investors," the spokesman said.