The Hornets stunned their opponents with a big win in their new manager's first home match in charge, scoring three of their goals in the final seven minutes of the contest.
Watford had won just one of their previous 12 top-flight games since last November, costing Marco Silva his job in Hertfordshire, and Gracia is hopeful that Monday's triumph can act as a turning point in their season.
"You always work for nights like this, but this is unforgettable," he told reporters after the match. "I am very proud of my team and very happy for our fans to enjoy a great victory.
"I am very happy because of this performance, really proud of the work of my players. The team worked very hard and we defended well. Chelsea are a very big club. It is important not only for the three points but we needed that for the motivation and to believe in our work."
Watford ended a 13-match winless run against Chelsea, which stretched back to a 1-0 victory in September 1999.