The 28-year-old striker has started just five league games for Forest since joining them from Millwall in December 2004.
He was not involved on Saturday as Gillingham met the two times European Cup winners at Priestfield - with Forest running out 3-1 winners.
However, Harris, who scored 98 goals in 268 league and cup appearances during his time at Millwall, is likely to make his Gills debut at Brentford on Monday.
He replaces Steve Claridge, who joined Bradford last week following a none-week stay at Priestfield.