Plunkett's career was revived under Gillespie's watch after he signed from Durham in late 2012 after a difficult period following the loss of his England place. He became a key part of Yorkshire's impressive pace attack and earned a recall to England's Test side in 2014 before finding a place in the one-day set-up in the post-World Cup rebuilding under Eoin Morgan, Trevor Bayliss and Paul Farbrace.
Initially it was a bit-part role last season and he then endured a frustrating winter where he did not feature in the one-day series against Pakistan and South Africa before belatedly being called into the World T20 squad when Steven Finn was injured. Once he made the XI he impressed with his hostility and that has continued with this summer's one-dayers against Sri Lanka and Pakistan in which he has taken 13 wickets in seven matches.
The confidence he is now feeling in an England shirt stems from the rebuilding process he underwent with Gillespie. He remembered an early stint in the Yorkshire nets during the winter that he signed for the club - following the 2012 season where he had played just one Championship match for Durham - where he could barely get the ball down straight and wondered whether he would have a future.
"One of reasons I moved to Yorkshire is that he was a bowler himself and went through his ups and downs," he said. "One of the first net sessions I had with him was in the winter and I hit the side netting. I thought that was the end of my career at Yorkshire right then but he said don't worry about it, it is November and he pretty much said he just wanted me to run up and bowl fast."
Plunkett singled out the belief Gillespie had helped give him rather than focussing too much on the technical aspect of his bowling. It has been Plunkett's pace - which can top 90mph - that has made him an asset to England's one-day side even though he is no longer in the immediate Test match plans.
"At that time, he was what I needed. I was a bit raw, and struggling with my action," Plunkett said. "I thought it was all about technical. But it wasn't - it was just about backing myself.
"When I went there, that was what he picked up straightaway. He does other stuff with different people, but he knew what I wanted. He just said to me 'bowl fast', and he backed me. With that came confidence and performance.
"Whenever you have a bad spell he is always there patting you on the back and saying, 'no worries, mate, you can change the game next spell'. I take my hat off to him, I owe him quite a bit.
"I'm obviously gutted - because he was good for me, and good for the team. It's a tough decision that he's made. But I want to thank him - he's been great for me. He's brought me back to playing for England. So I wish him the best, and look forward to catching up when I get back to Yorkshire."
Yorkshire have gone out of both limited-overs competitions - the T20 Blast and Royal London Cup - at the semi-final stage over the last two weekends but a hat-trick of County Championship titles remains in view as they sit second in the table, five points behind leaders Middlesex.
"I hope we can make it three this year in the Championship," Plunkett said. "That would be a nice note to leave on. I'm sure there will be quite a few people trying to get that job."