The Yorkshire first-team coach made his comments after his side completed the opening half of their Championship fixtures yesterday with a draw against Nottinghamshire at Headingley.
Yorkshire sit second in the table, five points behind Somerset, after three wins, four draws and a solitary defeat in their opening eight games.
And Gillespie, who described the title race as “wide open”, with just 44 points separating the leading eight teams, insisted his side can only get better.
“I think we’ve performed well overall and our league position is pretty good, but I think we’ve still got at least 10-20 per cent improvement in us,” he said.
“When we’ve clicked, we’ve won games of cricket, simple as that. The games that we’ve won – one against Warwickshire, two against Northants – there was absolute belief and discipline among the players.
“But we need to do that more often for us to be sitting at the top of the tree, and I definitely think there is more to come.”
Gillespie’s view was supported by Yorkshire’s up-and-down performance against Nottinghamshire.
The home side bowled well to dismiss the visitors for 205 on the opening day, only to squander the chance of a sizeable lead by being ejected for 247 in reply.
Yorkshire reduced Nottinghamshire to 154-5 in their second innings yesterday – effectively 112-5 – to create a real chance of forcing the win.
But they were unable to take it as James Taylor (96) and Chris Read (75) added 115 for the sixth-wicket, Nottinghamshire going on to 335-8 declared before Yorkshire – set a notional 294 in 39 overs – ended on 53-3.
“I think we were probably 70-80 runs shy with our first-innings batting,” said Gillespie, whose side return to Championship action on Monday against Sussex at Arundel.
“We should have posted over 300 runs – that would have put Notts under immense pressure – and that was an area where we failed to drive home the advantage.
“Then, when we got the fifth wicket on day four, Read came out at a pivotal moment and played a good innings and had a great partnership with Taylor.
“They probably got Notts out of the woods, to be honest, and all credit to them.”
Yorkshire’s failure to drive home the advantage highlighted several things.
Most obviously, they are somewhat less formidable without pace bowlers Liam Plunkett and Ryan Sidebottom, the former on England duty and the latter expected to be back for the Championship match against Warwickshire at Edgbaston on Sunday week after a hamstring injury.
In such circumstances, Yorkshire really needed their other England bowler, Tim Bresnan, who is currently out of the Test side, to stand up, but he looked ordinary as the Nottinghamshire lower order picked him off professionally.
Bresnan finished with 4-112 and would have had the rarity of a five-wicket haul had Aaron Finch not dropped former Yorkshire pace bowler Ajmal Shahzad at third slip, but such riches would have flattered Bresnan.
Steve Patterson was by far the best bowler on his way to 4-57, while leg-spinner Adil Rashid made no impact.
Read and Taylor played superbly. Read went to 50 from just 55 balls and the only chance he gave was on 53 when Yorkshire could have dismissed either him or Taylor off the same delivery.
Patterson bowled it and dropped a very tough return chance one-handed to his right when Read hit back a fierce drive towards the Rugby Stand.
The ball ricocheted to mid-off, from where Bresnan threw to the bowler’s end as Read scampered away for a single when a simple throw to wicketkeeper Jonny Bairstow would have run out Taylor.
In that moment, with the second new ball less than two overs old, Yorkshire’s last chance of winning effectively disappeared.
Taylor deserved a century only to depart to a rank leg-side half volley off Bresnan that begged to be hit for four only for the batsman to smash it straight to square-leg, where Rashid took a terrific catch.
Read pulled Bresnan to the same fielder to end a seventh-wicket stand of 45 from just 38 balls with Peter Siddle, who thumped an unbeaten 48.
In their second innings, Yorkshire lost Alex Lees to a catch behind, Jack Leaning to a top-edged hack to mid-on, and Adam Lyth to a catch at leg slip as they briefly flirted with fiasco. But Andrew Gale and Bairstow averted that after the visitors’ declaration.
Scorecards: Page 23.