The Aston Villa loanee was not meant to take the free kick, which he scored.
Joe Mason fired the visitors into a 1-0 lead on 58 minutes at the City Ground, but a beautifully-struck free kick from the Aston Villa loanee 10 minutes later proved enough to earn a point for Forest, who are now unbeaten in four Championship outings.
Ex-Arsenal man Lansbury was penciled in for set-piece duty, but the 23-year-old defied his team-mate, and manager, but vindicated himself in stunning fashion.
Although Williams admits it was a "superb" effort from the midfielder, the caretaker manager revealed that he initially had a "moan" when Gardner insisted on taking it.
"It was a superb free-kick from Gary Gardner – and I did moan that Henri was not taking it, before he struck it," admitted Williams to the Nottingham Post.
"But we give ownership to the players over that kind of situation. Henri was meant to take it, but, if Gary felt he had a good chance of scoring, then there is no problem – Henri was clearly big enough to think of the team. Happily so.
"The free-kick was the highlight of the game – that and the way the fans clapped the players off the pitch at the end. They were fantastic for us."
The goal aside, Williams might be disappointed that his boys did not get the victory, having played well enough but they appeared to lack something of a cutting edge.
Furthermore, it came one week after they beat Fulham 3-1 at Craven Cottage to end an eight-match winless run.