His team is spending 18 days on site at London’s Ranelagh Gardens to create a show garden for tourism body Welcome to Yorkshire ahead of the event opening for five days on May 21.
Today, two pairs of canal lock gates once used on the Huddersfield narrow canal and donated by the Canal & River Trust arrived to form the garden’s centrepiece.
Each gate weighs 1.5 tonnes and they will be installed as part of a working canal, alongside a towpath and a lock keeper’s lodge.
Mr Gregory believes the design is one of the most ambitious being attempted at this year’s show due to its sheer scale - and he is confident it will be a star attraction.
"I know this particular garden will go down in the annals of history as an iconic piece of work.
“I just want to see people’s faces and see their jaws drop,” said the designer, whose 2018 Yorkshire Dales-themed entry for Welcome to Yorkshire won Gold, as well as the People’s Choice and Best Construction awards.
Speaking to The Yorkshire Post, he admitted his reputation was on the line after last year's success.
“I wanted to really test myself and do something extraordinary," Mr Gregory said.
“The water is going to be pretty ‘shock and awe’. Last year’s garden had two pumps, this year there’s nine. There’s going to be water coming from everywhere. It’s not so much a garden as a piece of theatre."
As for the scale of the garden, he said: “I’ve never worked on a garden that is so high at the back and so low at the front. At the back it rises up to 1.8 metres and on top of that we’re planting trees of up to 7ft tall.
"The canals of West Yorkshire climb and I have to get that across in this small space. Everything needs to be in proportion.
"The front of the garden will be wild, there will be nettles, brambles and wild flowers and these huge willows.
"All the pretty stuff will be around the lock keeper’s lodge because in my mind, the lock keeper is a keen gardener looking after the locks.
"There will be this man made meadow across the division of the cultivated stuff and there will be the wild bit across the front and a woodland patch.
"What I’m trying to do is create little moments, it will be very three-dimensional. I’m falling in love with it.”
Work started on site last Tuesday and Mr Gregory said: "We are just over a third of the way through the build and it’s going like a dream. We have had a few technical issues with some of the designs which you always get but that’s part of the adrenaline kick.
"It’s starting to look a lot like West Yorkshire. Already the detail is staggering. It couldn’t be any better and I think the public are going to love it and the cameras will as well."
The inspiration for his design is the stretch of canal between Huddersfield and Marsden, including Standedge Tunnel.
"It’s beautiful there and very ‘industrial heritage’ with the old mills.
"My message to the public, to RHS, is these canals were motorways of the time, they were gritty and dirty, and now we have this fantastic environmental legacy."