Gruebel left the meeting held at the bank's Singapore office in a chauffeured silver limousine but he made no response when journalists asked "were you fired?" and "did you lose your job?" an AFP reporter on the scene said.
Other executives left the office one by one, with security people blocking access.
In Zurich, a UBS spokesman confirmed: "For the day, the meeting is over."
He would not give details about the contents of the meeting or if further meetings would be held over the weekend.
Swiss media reports said Gruebel was expected to seek a vote of confidence during the meeting, held as the bank faced pressure from shareholders and some Swiss politicians over a rogue-trading scandal that caused the lender to lose $2.3 billion.
UBS Executive Board Member Alex Wilmot-Sitwell, the co-chairman and co-chief executive of UBS Asia-Pacific region, said the three-day meeting had ended but did not give any further details, Dow Jones Newswires reported.
The long-scheduled meeting was held in the run-up to the Singapore Grand Prix on Sunday.
UBS is a major sponsor of Formula One racing and a racing simulator with the UBS logo adorning the lobby of the bank's building, located on the fringes of the street circuit being used in Sunday's night race.
On Tuesday, the Government of Singapore Investment Corp (GIC), UBS' biggest shareholder, issued a rare public rebuke to the bank for lapses that led to the losses.
"GIC expressed disappointment and concern at the lapses and urged UBS to take firm action to restore confidence in the bank," the cash-rich sovereign wealth fund said in a statement.
"GIC sought details of how UBS is tightening the control environment and looks forward to the conclusions of on-going investigations."
Meanwhile in London, the trader accused of causing the massive losses was on Thursday remanded in custody for another month.
Kweku Adoboli, 31, faced an additional charge of fraud as he made a brief appearance in front of a packed City of London Magistrates' Court but did not enter a plea or appeal for bail.
"He is sorry beyond words for what has happened," defence lawyer Patrick Gibbs told the court.