The sources told Reuters newsagency that Chinese companies were among several investors eyeing Opel/Vauxhall. Others were European industrial groups and sovereign wealth funds.
One source also told Automotive News Europe that GM is talking with "credible" potential investors in the brands including Chinese groups.
Four financial and two strategic investors are in talks with GM to buy a possible majority stake of Opel, which would involve an investment of at least 500 million euros (US$649 million) in direct equity, a banker close to the negotiations told Reuters.
The source confirmed a report in the Financial Times that half a billion euros was the absolute minimum expected.
None of Germany's carmakers nor Italy's Fiat SpA, which had been repeatedly named by the media as a potential partner for Opel, was involved in the sale process, the Reuters source said.
The banker also said well-known private equity firms were among the parties interested in Opel. However, they would have to rely on traditional capital gains from a successful restructuring of Opel rather than a classic leveraged buyout.
Sovereign wealth funds, by comparison, could be more interested since they rarely seek management control and mainly purchase minority holdings.
GM is carving out German-based Opel and its British sister brand Vauxhall into a separate unit and is seeking outside investors for the unit.
GM CEO Fritz Henderson said on Friday that confidentiality agreements have been signed with six groups interested in buying part of Opel/Vauxhall.
To prevent future liquidity problems, Opel has requested loan guarantees of 3.3 billion euros (US$4.34 billion) from governments in European countries where it has factories.
German Chancellor Angela Merkel has made the approval of any such guarantees from Germany conditional on finding outside investors.