It has been reported that Derby are considering making a £500,000 bid for the 29-year-old striker.
Gregory has scored five goals and provided two assists in 23 Championship starts for Millwall so far this season.
The former Mansfield Town player is a decent striker, and it would not be a bad idea for Derby to sign him, especially if they need not pay more than £500,000.
The Rams are going strong in the Championship at the moment and look set to finish in the playoffs at least.
Gary Rowett’s side are currently second in the league table with 53 points from 28 matches, points clear of Cardiff City and Bristol City, who are third and fourth respectively.
Meanwhile, Millwall find themselves 15th in the Championship table with 34 points from 28 matches.