A senior figure in the coalition said the Labour Party, one of Georgia's biggest opposition groups, was no longer in the coalition and might field its own candidate, the Reuters news agency reported.
"It won't be an ordinary election," Gachechiladze said.
"It will be against violence, it will be against injustice and it will be against the institution of the presidency."
One of the opposition's main policies is to scrap the post of president.
'A good candidate'
Salome Zurabishvili, a former foreign minister, is to run as Gachechiladze prime minister.
The French-born Zurabishvili said: "Levan is a good candidate [for president], a businessman very well known by the public and very popular, who everyone knows has absolutely no presidential ambitions.
"I bring my national and international experience and domestic popularity. Our tandem has very serious chances.
"We have real chances on the condition that the climate of political repression is lifted."
Earlier on Monday, members of the Labour party read a statement on television saying they were considering putting forward a candidate of their own.
"Today's statement from the Labour party means it has split from the coalition," Koba Davitashvili, one of the coalition leaders, said.
Without the Labour party, the opposition coalition is reduced to nine parties.
In previous elections, the Labour party has taken about 10 per cent of the vote.
Protest leader
Gachechiladze, who is not affiliated to any party, has been at the forefront of opposition protests this month against Saakashvili, which ended when police used tear gas on protesters.
He and had been one of dozens of activists who started a hunger strike on the steps of parliament.
Saakashvili later declared a state of emergency, as riot police cracked down on protesters in the capital, and then called for snap elections as a concession to the opposition.
Badri Patarkatsishvili, a media magnate and Georgia's richest man, has also said he will be running in the elections.