Low, 29, will complete his switch to Whaddon Road if he passes a medical after agreeing a two-year deal with the Robins.
"I'm delighted that Josh has joined us because he's a player who still has great potential," Downing told Town's official website.
"He's played at this level and higher and hopefully this move will help to galvanise his career. He needs a fresh challenge and we can give that to him.
"Josh is a traditional right winger who can take people on and provide crosses. He's six feet tall and a powerful runner who can supply the forwards and score a goal or two himself."