Wales international prop Gill, who signed for Sarries in 2009, joins the likes of Schalk Brits in agreeing new deals with the Aviva Premiership champions.
Director of rugby Mark McCall is delighted that the former Cardiff Blues player has committed his future to the Vicarage Road outfit.
"Rhys has earned a reputation as one of the most effective loosehead props in the Premiership, and he has richly deserved his recent inclusion in the Wales squad for the Six Nations," he said.
Gill added: "Saracens were the club that showed faith in me three years ago, and it follows naturally that I show faith in the club now."
McCall also hailed the deal signed by 21-year-old academy product George, who said that he hopes he will stay at the club throughout his career.
"Jamie knows how much he is valued at this club," McCall said of the hooker.
"He's a hugely talented young hooker and a natural leader with massive potential in the game, and we are delighted he has chosen to keep working hard to fulfil that potential at Saracens."
South African-born prop Du Plessis extends his stay at Saracens after joining them from Northampton a little under two years ago.
On 29-year-old former England prop Stevens, McCall said: "Matt is widely recognised as a world-class player, performing week in week out at the very highest level.
"We are fortunate to have a player of such quality within our squad, and we are very pleased he has agreed to extend his contract."
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