The winger scored deep into stoppage time to secure an undeserved point after Dons had dominated all game.
Top scorer Matt Fryatt gave the Foxes an early lead but Peter Leven scored in each half to seemingly give Dons all three points.
But then to break Dons hearts and keep Leicester's amazing unbeaten run, going Gradel curled home from the edge of the box.
Dons got off to a nightmare start as Fryatt, who caused all sorts of problems on the first day of the season, scored an easy opener.
He received the ball with his back to goal and exquisitely turned Miguel Llera, the ball embarrassingly going through his legs as he was left on the floor with the clinical striker then coolly tucking the ball away.
Things looked ominous after that, but to Dons' credit they came back courageously and dominated the rest of the half.
And just five minutes later Dons had a great chance as Aaron Wilbraham played through the gallivanting Luke Chadwick, who with the goal gaping and one-on-one somehow fired wide.
They continued to pile forward and Jason Puncheon fired a shot over and then almost got on a Wilbraham cross after a lightning quick break.
Jemal Johnson then fired high and wide from the edge of the box as Dons' wastefulness looked like it may cost them.
Then with ten minutes to go until half-time the pressure paid of as Leven finished a slick move.
Puncheon turned superbly and played the ball into the feet of Wilbraham, whose lay-off for Leven was inch-perfect for him to hit a screaming left-foot shot into the bottom far corner.
Then on the stroke of the half-time they thought they had taken the lead as Johnson's corner was only cleared to Puncheon, who fired in a volley that was cleared on the line by Matt Oakley.
Dons started the second half with the same intensity and although they didn't create a clear-cut chance were outclassing their visitors.
Leicester's only real pressure came from a quick free-kick that was played to Lloyd Dyer, but his cross to Howard by surprise who blasted miles wide.
Then just past the hour mark Leven played the ball into the feet of Chadwick, who dummied superbly for Wilbraham, who played the ball to the advancing Leven.
The Scot tried to dink the ball over him, but hit the body of Mark Bunn, luckily it rebounded into his chest and trickled over the line.
Then from the kick-off Dons poached the ball and Puncheon sprinted through the middle and, with options either side, hit a low shot that was well held by Bunn.
Leicester just couldn't get into the game and their only option seemed to be playing the ball into Howard or Fryatt and hoping it stuck.
Dons continued to attack though and Johnson cut in well and hit the side-netting with a right-footed shot.
With eight minutes to go Leicester finally registered a shot on target as substitute Gradel hit a weak shot from the edge of the box that Willy Gueret smothered easily.
Then minutes later came a glaring miss as the ball fell nicely to Andy King on the edge of the box only for the Welshman to fire over.
But then, six minutes into added time, Gradel curled over the wall and into the net despite the efforts of Llera on the line.