The left-hander hit five sixes and two fours to clinch an unlikely win, a target of 184 being reached with two balls to spare in a thriller. Skipper Michael Klinger contributed 45, while both Alex Gidman and Ian Cockbain made 27.
Essex, who had already reached the last eight, posted 183 for seven after losing the toss, Ravi Bopara profiting from being dropped three times to make 51 and Ryan ten Doeschate smiting 27 off nine balls at the death. Benny Howell was the most successful bowler with 4-26 from his four overs.
It was only the second time in 12 group matches that Essex had suffered defeat and came against a Gloucestershire side who only had pride to play for, with no chance of reaching the quarter-finals.
The visitors made a poor start to their innings when Tom Westley was caught behind for a golden duck off the third ball of the game, bowled by left-arm spinner Tom Smith.
— Essex Cricket (@EssexCricket) July 20, 2014
But Mark Pettini and Kishen Velani responded with a stand of 42 from 28 balls before Pettini was caught at deep square-leg by Howell off Graeme McCarter. His 26 had featured five fours.
At the end of the powerplay Essex were 59 for two. The eighth over saw Bopara survive a caught-and-bowled chance to Howell on four and Velani, on 34, was held at long-on the ball after depositing a full-toss over midwicket for six.
Bopara was badly dropped for the second time on 19, Jack Taylor spilling the chance at short third man from a reverse-sweep, before going on to add 52 in 34 balls for the fifth wicket with Ben Foakes, who made 23.
Bopara had hit six fours and a six by the time he fell to Howell in the 18th over, the ball after being dropped for a third time by David Payne, the same fielder atoning on the deep midwicket boundary.
If Gloucestershire thought that was the end of their problems, Ten Doeschate had other ideas. The Essex skipper smacked two fours and two sixes in his cameo and 19 came off the final over, sent down by McCarter.
Gloucestershire openers Klinger and Gidman both hit straight sixes off the third over of their team's reply, bowled by left-arm spinner Tim Phillips, who had been given the new ball.
Post match autograph scrum! pic.twitter.com/i1ZlupkuTg
— Gloucestershire CCC (@Gloscricket) July 20, 2014
George Sharp was umpiring at both ends, with his partner Stephen Gale operating at square-leg having received a painful blow on the knee from a straight drive by Bopara during the Essex innings.
Young seamer Matt Salisbury conceded only three off the last over of the powerplay and it was not until the seventh over that Klinger and Gidman brought up their half-century stand.
Bopara broke the partnership on 71 by inducing a return catch from Gidman while conceding only two off the ninth over.
There was no keeping the England international out of the action as he then took a catch at deep midwicket off Phillips to send back Howell for a brisk 16.
Salisbury took a brilliant, low catch running round from short third man to remove dangerman Klinger, with Gloucestershire requiring 91 off 7.5 overs, but successive sixes from Dent off Greg Smith in the 14th over kept the home side's hopes alive.
Cockbain helped to add 70 in six overs before being bowled by Graham Napier at the start of the 19th.
Even then Essex looked favourites, but Dent was now flying and finished the game with two, two, six, four off the final over from Salisbury to secure the win.
Dent said: "It was time I did something for the team because I have had a lean season by my standards. There was a short boundary on one side and I always felt we had a chance because it didn't take much of a hit to clear it."
Essex skipper Ten Doeschate said: "We were carrying a few injuries and it was a bit of a jaded performance. I felt we should have scored more runs on that pitch and we are disappointed."