Gregan was told by Dennis Wise that he is free to leave Elland Road and a number of League One sides were keen to sign the 32-year-old.
He has been given permission to play for the Latics in their FA Cup 1st round match at Kettering on Saturday.
His last performance for United was the 1-nil defeat at Coventry in what proved to be Kevin Blackwell's final game as manager.