Reid, who bagged 18 goals for the Millers last season, looks set to tie up a move away from the Don Valley Stadium at any day, with West Brom his reported destination.
Gray, 28, was released by Leyton Orient at the end of last season having spent a large part of the campaign on the sidelines due to a cruciate ligament injury.
Robins told the Sheffield Star: "He has been training pre-season with his old club, Southend, but he hasn't got fixed up and we got into talks. With what appears to be the imminent departure of Reuben Reid, we need to do something, to bring someone else in who can also add that little bit of pace.
"We gave him most of the second half at York on Tuesday and had him down for Wednesday night at Mickleover as well before it was called off.
"There's no timescale put on how long he'll be here - it depends how long he wants to stay. He'll stay on with us this week and be here for Saturday's game against Manchester City. If he likes what he sees then he'll hopefully want to do something, but we'll see."