The Yorkshire first-team coach offered no excuses for the club’s dismal showing in this year’s competition, which has seen them lose eight of their 13 games and fail to reach the knockout stages.
Yorkshire play their final group match tonight against Birmingham Bears at Headingley (6.30pm start).
Gillespie wants his players to go out on a high, but he conceded the club need to rethink their T20 strategy for next summer.
“We pride ourselves on the standards we set and we simply haven’t been good enough in Twenty20 cricket,” he said.
“We therefore need a bit of a rethink as to how we go about it and that will happen in due course.
“It’s something we’ll be looking at moving forward because, without a doubt, it’s been unacceptable and we need to play better.
“To cut to the chase, we haven’t done the business, and we need to try to find some solutions.”
Yorkshire made a concerted effort to improve their T20 form this year after they failed to reach the knockout stages last summer too. They proved their commitment by signing two of the world’s best T20 batsmen in Aaron Finch and Glenn Maxwell, neither of whom have fulfilled expectations.
Finch managed just 76 runs in six innings with a top score of 33 before suffering a foot injury.
Maxwell, who will tonight go head-to-head with the New Zealand star Brendon McCullum, has managed just 229 runs in 12 innings with a highest of 92 not out, making more of an impact with his off-spin bowling than with the bat.
Although Yorkshire had a right to expect more from both of their overseas players, their T20 work has been collectively poor.
The batting line-up has failed to fire; of those who have played regularly, just Jack Leaning, Jonny Bairstow and Tim Bresnan have managed to average more than 30.
The bowling unit has struggled, too, specifically failing to contain opponents at the beginning of the innings and also at the death.
Bresnan, a former World T20 winner, has conceded over 10 runs per over, with 17-year-old pace bowler Matthew Fisher the club’s leading bowler, followed by Maxwell.
“The fact is, our senior players haven’t stepped up to the plate and done the job in this form of the game,” conceded Gillespie.
“Our best bowler has been a 17-year-old seamer in Matt Fisher, who has been outstanding, and our frontline batsmen haven’t really fired.
“The key areas for us have been our up-front bowling and our bowling at the end of the innings, as well as our consistency with the bat.
“We just haven’t had that, and that’s why we’ve struggled.”
Another reason Yorkshire have perhaps failed to live up to their billing as one of the pre-tournament favourites has been their refusal to compromise their performances in County Championship cricket.
Gillespie and the coaching staff have quite rightly prioritised the four-day competition, with Yorkshire odds-on to record back-to-back titles after their seven-wicket victory over Worcestershire on Wednesday sent them 34 points clear with a game in hand.
As such, Yorkshire have resisted any temptation to play Ryan Sidebottom in T20 to boost their challenge, while fellow pace bowlers Jack Brooks and Steve Patterson have made only seven appearances between them.
Yorkshire have blooded a number of youngsters, whose inexperience has perhaps been highlighted by the struggles of the seniors.
“Some people question us when we leave players out in T20, especially if there’s a Championship game starting the next day, or two days later,” said Gillespie.
“People perhaps question how much we want to do well, but I can assure you that we want to win every game.
“We’re very clear, though, that you can’t play 11 players across all three forms of the game. That just doesn’t happen.
“There’s not one team in the country that does that.
“You have to have a squad mentality, but you have to have that in Championship cricket as well.
“We’ve had changes of personnel in Championship cricket as well as in Twenty20, and we can’t blame that in any way, shape or form.
“The facts are, we’ve got a squad capable of competing in all forms of the game and we simply haven’t been good enough in the T20 Blast.
“I’m not going to make any excuses.”
Holders Birmingham have already secured a home quarter-final after beating Northants on Wednesday.
They lead the North Group after 10 victories from 13 games.
Yorkshire have made one change to the squad that lost by six wickets against Northants last Friday.
Bairstow’s selection for the third Ashes Test at Edgbaston next Wednesday means that reserve wicketkeeper Andrew Hodd returns to the team.
Academy all-rounder Matthew Waite is in line to play his second senior match for the county after an impressive display at Wantage Road.
Waite hit an unbeaten 14 and bowled four useful overs.
Yorkshire squad v Birmingham: Carver, Coad, Craddock, Fisher, Gibson, Hodd, Leaning, Lees, Maxwell, Plunkett, Rhodes, Wainman, Waite.