Former Gulls striker Benyon spent three and a half years at Plainmoor before leaving for Swindon in 2011 having scored 39 goals in 125 appearances.
The 25-year-old returned to the Devon club on loan from Southend at the end of last term and boss Alan Knill has moved quickly to make the deal permanent following his release from Roots Hall.
"He was brilliant at the end of the season," Knill told the club's official website.
"I think everyone saw that the longer he was here, the better he became and the fitter he became."
Former Rotherham right-back Tonge will also play in blue and yellow next season after completing a deal following discussions with the club last week.
The 28-year-old had been a regular for the South Yorkshire side until a persistent injury limited his appearances last season during the Millers' promotion campaign.