Vauxhall's pension scheme is one of the largest in the UK, with 15,000 members.
Pensions expert John Ralfe said Peugeot owner PSA would not want to touch it "with a barge pole", saying he thought it had a deficit of about £1bn.
Half of the members were pensioners, Mr Ralfe told the BBC's Today programme.
The proposed sale will be discussed in the Commons on Monday afternoon after the Speaker granted an urgent question on the matter to the Labour MP for Ellesmere Port and Neston, Justin Madders.
Meanwhile a Downing Street spokesman confirmed that Prime Minister Theresa May was planning to meet Carlos Tavares, PSA chief executive, but a time and date for the meeting had not yet been fixed.
According to company filings to the end of 2014 - the latest available - the Vauxhall pension scheme had assets of about £1.8bn but liabilities of about £2.6bn, leaving a deficit of £840m.
Since then record low interest rates have hit the returns on government debt in which big pension schemes invest heavily, so the deficit had probably grown, Mr Ralfe said.
Last week it emerged that PSA Group, which also makes Citroen cars, was in talks about taking over GM's loss-making European business, Opel.
Mr Ralfe told Today the size of the deficit was a "major issue for the takeover". "At best it's a stumbling block, at worst it could be a deal breaker," he said.
He said he was clear that PSA would not want to take on the pension scheme so they would only buy the operating assets, including the plant and the Vauxhall brand, leaving the pensions with General Motors UK.
"The trouble with that is that would then be a company with no assets, so what would have to happen ... is that General Motors US would have to issue a guarantee for that UK company," Mr Ralfe said.
PSA, which already works with GM in Europe on several projects, said a takeover was among "numerous strategic initiatives" being considered.
Any deal would involve Opel's UK arm, Vauxhall, which employs 4,500 staff at plants at Ellesmere Port and Luton.
Unite union leader Len McCluskey is also due to meet Mr Tavares this week to discuss the deal and any impact it might have on jobs.
Mr Ralfe said he did not think the Vauxhall pensioners needed to be worried about their position, but said that while politicians were "running around all over the place asking about jobs, they should also be running around asking about pensions".
Plans to sell the European arm of General Motors - including Vauxhall - to France's PSA Group could be derailed by the deficit in GM's UK pension scheme, an expert says.