Darboe's comments are an early sign that opinion is shifting against Gnassingbe who took power in 2005 on the death of his father who had ruled since 1967.
Togo is enduring a political crisis in which at least 10 people have been killed in protests since August.
"I think the African Union and ECOWAS should engage Togo and persuade the president to step down," Darboe told Reuters this weekend, adding that other countries need to step in. "When it goes against accepted norms I don't think it should be treated as an internal affair."
Currently, Gambia and Togo are the only countries in the 15-nation ECOWAS bloc without time limits on mandates. ECOWAS sought to make this law across the region in 2015 but Togo and Gambia voted against it, although Gambia is now changing its constitution.
Gnassingbe is currently in his third term. Opponents have organised mass protests since August amid growing frustration over delays to constitutional reform and are seeking an end to his rule.
Security forces have repressed protests, firing on demonstrators, and raised the chances Gnassingbe could be toppled as a result of popular anger, as happened in Burkina Faso in 2014.
Togo's referndum
Criticism of Gnassingbe's government by foreign powers and other ECOWAS countries has so far been muted, perhaps because the Togolese leader holds the ECOWAS chairmanship until June 2018.
Darboe was the main opponent of former President Yahya Jammeh during his 22-year dictatorship which ended in January as ECOWAS troops marched into Gambia to uphold an election result.
He was imprisoned for most of last year for joining a protest for reform and said he thought talks with Gnassingbe should begin after Togo's planned referendum on term limits.
The Togolese measure, intended to quieten critics, has failed to appease the opposition who say that it would not be retroactive, meaning that Gnassingbe could stay until 2030. In a statement, the AU and ECOWAS have welcomed the proposed reforms, called for peace and encouraged dialogue.
Gambia's government plans a constitutional review to cap a president's tenure to just two four- or five-year terms among other reforms, Darboe said. "No national assembly should have the power to nullify that law as we've seen elsewhere in Africa," he said.
