Dunn turned creator to set up Craig Conway to give Rovers the lead in the second half but a Troy Deeney penalty and a late breakaway goal from Cristian Battocchio looked to have secured a home win until Gestede's late leveller made it 3-3.
The Hornets have not won away since October but they have had no such problems at home under Beppe Sannino and it took only four minutes for them to take the lead.
Daniel Pudil started things off with a throw-in from the left which Deeney hooked into the danger area and Cassetti got in front of his marker to steer a left-footed half-volley past Paul Robinson to net his first Hornets goal.
Blackburn responded well to that setback though, and were almost level within a few minutes when Dunn put Gestede through one-on-one with Manuel Almunia but the striker was denied by a fine save.
Watford had gone almost 11 hours without conceding a goal at home but that run was to end in the 27th minute. Almunia sought to punch clear a free-kick but failed to get sufficient distance on the effort and the ball dropped to Dunn on the edge of the penalty area who guided a first-time effort through the crowded goalmouth and into a largely unguarded net.
The home side thought they had gained the upper hand for a second time within two minutes of the restart when Ikechi Anya got in behind the defence and squared for Deeney to net from close range, only for the goal to be ruled out for offside.
Another Watford chance came and went soon after when Anya was released but he dragged a shot wider of Robinson's far post.
With their next good opportunity, though, Rovers took the lead and Dunn turned creator on this occasion as he showed some superb close control to weave his way into the area before setting up Conway to calmly apply the finish from eight yards out.
But Blackburn's advantage was to last just seven minutes as, after Alexander Merkel had won the ball back on the Hornets right, Anya embarked on a run before his attempted cross was blocked by the arm of Todd Kane.
Referee Darren Sheldrake had no hesitation in pointing to the penalty spot and Deeney calmly did the rest, sending Robinson the wrong way to net his 17th league goal of the season to make it 2-2 in the 71st minute.
The Hornets looked to have secured their seventh straight home win two minutes from the end of normal time when they sprung the counter-attack and Anya put Battocchio through on goal and the substitute kept his composure to net what looked to be the winner.
But Blackburn were to salvage a point in stoppage-time when Tommy Spurr's cross from the left was met by a towering header from Gestede who beat Almunia with the help of the far post.
- Sports & Recreation
- Troy Deeney
- David Dunn
- Manuel Almunia
- Cristian Battocchio
- Craig Conway
- Marco Cassetti
- Ikechi Anya