It had been used as a residential school for children with behavioural difficulties since the 1950s, and its appearance reflected the spartan educational climate of the era.
Hundreds of beds were still inside the vast dormitories that had once been the Victorian house's grand bedrooms, while the sunken rose garden and ornamental ponds in the formal grounds were overgrown and concreted over.
Yet Gary, a former washing machine engineer who has become one of Doncaster's most visionary businessmen, abandoned his plans to convert the hall into flats after just one viewing, and decided instead to realise its potential as a country house hotel.
Old photographs supplied by former staff gave him an insight into what the house had looked like in its heyday, after the Streatfield family moved in in 1883. They lived at their new residence on the medieval Shooters Hill hunting estate until 1937, when their daughter Annette died and the hall became first a Catholic missionary training college and later a wartime base for the Royal Veterinary Corps.
Rossington Hall once had an entrance hall, dining room, library, conservatory, drawing room, two sitting rooms, billiards room and 22 bedrooms, as well as gardens, 250 acres of land and a stable block.
Although small by the standards of country houses - the Streatfields were only minor gentry - it was a thriving estate which provided employment for local people.
It was a hive of activity once again when Gary's loyal staff arrived on the site in 2011 and began gutting the school, which had closed three years earlier. They opened the hotel's restaurant just a year later.
"When I first saw it, it was cold, damp and institutional. There were gloss walls and tiled floors, but surprisingly no damage or graffiti that might have been caused by the children. There was a burst pipe in the dining room and water had come through the ceiling."
Gary's childhood memories of the school are of a forbidding, isolated place cut off from mainstream society.
"Anybody my age who grew up around here would say it was a school for 'problem' children - we were told not to go down the road to it. I never dreamt that there was such a nice building at the end of it. Back when it first became a school, nobody wanted or appreciated big houses, and it could easily have been knocked down.
"I only saw it once before I bought it, and I had planned to turn it into housing. But when I saw inside, I knew immediately that it had to be a hotel - I couldn't subdivide it, it needed to be kept in one piece and restored. The original staircase was still there and it looked beautiful."
His team spent months ripping out beds, dividing walls and wall-mounted electrics, and gradually the house gave up its 19th-century secrets.
"I wanted to get a full understanding of the original layout, so we could feel the ambience of the 1880s. We used local craftsmen who can turn their hand to anything.
"One of the main challenges was working out how each of the rooms had been laid out. We had a photo of the drawing room taken in the 1950s which showed two marble fireplaces - they'd been bricked up and we opened them up again. They'd been smashed to bits but we managed to produce exact copies using the picture, which was quite satisfying."
Gary's intention was always to open the building before its refurbishment was complete in order to generate an income to fund further work.
In 2012, staff began serving meals in the dining room at weekends. At that point, there was no working central heating system and they relied on open fires for warmth. By 2015, they had opened seven bedrooms, and there are now 21.
"We have a lot of customers from the early days who keep coming back and enjoy watching it develop, and we do more weddings now. We get a lot of business guests, which we didn't anticipate at first."
Celebrity guests to have stayed at Rossington Hall include actor Sir Ian McKellen, performer Sheridan Smith - who is from the area and got engaged at the hotel - and comedian David Baddiel.
"We're one of the nicer hotels in Doncaster and I think word has got out among the VIPs. We're a little bit different, a hidden gem and very discreet - we want to blend into the background."
One of the major challenges Gary faced was rejuvenating the grounds, which had become a wilderness. Most areas had been neglected for 50 years and left to grow wild. A yew hedge had been allowed to reach 60ft in height and destroy the rose garden, while ponds had been covered up to prevent children falling into them.
"Once we started pulling trees up, we found features we didn't know existed, which was a nice surprise. We had a photo showing a young girl in the rose garden which we were able to use to recreate it."
The next phase of the project is a conversion of the single-storey 1950s classroom extensions into more bedrooms and a sports wing with gym, golf simulator, badminton court and table tennis.
Yet Gary has even more ambitious plans to eventually turn the estate into a luxury spa resort.
"We want 40-50 bedrooms and a spa facility. We've got 250 acres here - some of the land is let to tenant farmers, and the Northern Racing College have 60 acres and the stables. They have a long-term lease and they have been pretty much born and bred here, so we would like them to be here for a long time yet - it is lovely to see the stables being used for their original purpose.
"It has been a dream project for me so far."