The 33-year-old has made 33 appearances for the Reds this season, scoring five goals.
Dons manager Neal Ardley told the club's official website: "I played with Gary at Millwall and I know what a good player he is.
"He has had a bit of a lean spell at Crawley but that has not detracted from his overall play.
"With his goals record and the character I know he is, everything added up to make it the right decision to bring him to Wimbledon."
Wimbledon are currently bottom of League Two and are one point adrift of safety, after playing 27 games this season.