The Senegalese foreign ministry released a statement after the meeting saying that Mugabe had agreed to stand for a run-off "in good faith and the firm will to accept the will of the people delivered in a free and fair election".
The statement said that Mugabe had also assured Gadio that he would "unhesitatingly accept the results of the second round and urged the opposition to take the same approach."
Zimbabwean election officials told a closed-door meeting in Harare on Thursday that opposition leader Morgan Tsvangirai had won 47.8 per cent and Mugabe had got 43.2 per cent of the vote, several sources present at the talks said.
But the opposition Movement for Democratic Change (MDC) party presented its own figures claiming Tsvangirai had won 50.3 per cent, just scraping past the threshold needed to avoid a second round run-off, the sources added.
Tsvangirai, whose party wrested control of parliament from Mugabe's ZANU-PF party in legislative polls also held on March 29, said he won a "decisive" victory and doubted the credibility of any official results given the delays.
Thursday's meeting came a day after government sources said that Tsvangirai had won the poll but not by enough to avoid a second round run-off.
Tsvangirai has previously said that the MDC, which he leads, would reject any run-off.
If he refuses to take part in a run-off, Mugabe will be declared the winner.
On Thursday, Tsvangirai told France 24 television that post-election violence had made it impossible to hold a second round run-off.
"How can you have a run-off when Mugabe over the last month has been unleashing violence, death squads and violence against our structures," he said from Johannesburg in neighbouring South Africa.
The MDC claims that at least 20 of its supporters have been killed in post-election violence.
Human rights groups have accused the government of using the army and so-called "war veterans" of using the delay in publishing the results to intimidate opposition supporters before a possible run-off.
The government has dismissed the accusations.