Following talks in Brussels with his Italian counterpart Pietro Lunardi, he said the two sides would begin immediate work on setting out a three-stage plan to open the tunnel to all traffic.
The heavily used route under the Alps was closed three years ago after 39 people died when a lorry caught fire inside. It was re-opened, but only to cars, earlier this month.
Italy said France was being unfairly obstructive in not letting all traffic through the tunnel, which is a vital trade link for Italy.
The country claims to have lost more than $2bn in trade due to the closure.
Click here to see a map of trans-Alpine tunnels
However, the French had raised safety and environmental concerns as a reason for not allowing heavy goods traffic back into the tunnel straight away.
Opposition
Mr Gayssot said Italy had agreed to his proposal to phase in the traffic to allow for the safety of the tunnel to be closely monitored.
In return, France will set a specific date, within a
week, for when large trucks can start using the tunnel.
Heavy trucks would be back using the tunnel by the summer, Mr Gayssot said.
"Our aim is to transfer 35% [of truck
traffic] from the Frejus tunnel [the other road tunnel between the two countries] to Mont Blanc," he said.
Local people have been strongly opposed to the return of lorries to the tunnel, arguing that the air had become cleaner during their absence, and that the snow near the tunnel entrance was no longer stained black.
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