'In the banana fields there are more dead bodies than bananas,' said Mr Gerin, who with his Mexican wife, Gloria, has sought refuge with advancing rebels of the Rwanda Patriotic Front (RPF) in Gahini, 60km (40 miles) east of the capital, Kigali.
Rotting victims of the mass killings litter the roads into Gahini village, lie in the fields and spill from hut doorways. The stench of death is inescapable.
The Gerins' nightmare began in the hours after the Rwandan president, Juvenal Habyarimana, along with his Burundian counterpart, was killed in a rocket attack on his plane on 6 April.
Mr Gerin said supporters of the hardline Hutu Coalition for the Defence of the Republic and Habyarimana's party came to threaten him at his ranch near Rusomo, 150km from Kigali. 'We spent 20 days in the bush after Interahamwe came and we fled,' Mr Gerin said, adding that he did not know whether his 50 staff at the ranch were alive.
Rusomo became the main crossing-point for up to 250,000 refugees into neighbouring Tanzania last week, until the Rwanda rebels occupied the frontier.
The Gerins are being evacuated through rebel-held territory to Uganda and then Europe, but Mr Gerin has vowed to bring to the world's attention what he calls the genocidal killings and the plight of Rwanda's civilian survivors. 'Something must be done quickly to help these people.'
According to Mr Gerin and most other eyewitness reports, the Interahamwe militias and the army are responsible for the massacres. Their victims are Tutsis or Hutu opposition-party supporters. The UN estimates 200,000 people may have been killed this month. Witnesses said the killers planned their butchering, herding people into confined areas or waiting for them at road-blocks.
Some aid workers have said the RPF was also guilty of civilian killings since it launched an offensive on 8 April in what it said was an attempt to restore order and end killing. No evidence of this has been published, but on Friday the RPF military commander, Major-General Paul Kagame, said his forces were killing militias members when they were caught. 'These are murderers. That's what they deserve,' he said.