Jose Adolfo Macias, known as “Fito” is accused of sending death threats to Ecuador’s murdered presidential candidate, Fernando Villavicencio.
The anti-corruption campaigner was shot three times in the head leaving a campaign rally on Wednesday.
Before his murder Mr Villavicencio said he had been threatened by Fito.
“If I continue… mentioning Los Choneros [the gang], they are going to break me,” he said.
His death has shocked a nation that has largely escaped the decades of drug-gang violence, cartel wars and corruption that has blighted many of its neighbours. Crime has however shot up in recent years, fuelled by the growth of Colombian and Mexican drug cartels.
On Sunday, his Construye party announced Christian Zurita, another journalist, would replace Mr Villavicencio as its presidential candidate.
This was a reversal from Saturday when the slain politician’s running mate, Andrea Gonzalez, had been named.
Mr Villavicencio’s widow Veronica Sarauz said she held the state responsible for her husband’s death that she was unhappy Ms Gonzalez had been named as her husband’s replacement.
Mr Villavicencio’s campaign focused on corruption and drugs. He was one of the only candidates to allege links between organised crime and Ecuadorian government officials.
The day before his assassination, he complained to the Public Prosecutor’s Office about alleged irregularities in oil contracts negotiated during former president Rafael Correa’s administration which had cost the country US $9bn (£7 bn).
Six Colombians have been arrested in connection with the murder, while a seventh was killed in a shootout. The authorities have not said who hired and paid the hitmen.
Gang leader Fito had been held in Prison 8 in Guayaquil since 2011, and videos shared by security forces showed him handcuffed in his underwear as security forces moved him to another facility.
Ecuador’s President Guillermo Lasso said Fito had been moved to La Roca, a 150-person maximum security prison that is part of the same complex.