Owned and developed by Malaysian-based YTL Hotels, the 99-bedroom property is one of the most anticipated hotel openings of 2015.
A central feature of the hotel will be the Spa Village Bath, where guests will be able to bathe in natural mineralised waters at a constant temperature of 47Â°C during a one-hour bath circuit, ending in a water ritual ceremony.
Guests staying in the spa suite and two spa bedrooms will be the first in the UK to enjoy direct access to the thermal waters via a second tap in their own bathrooms.
The Grade II-listed building was home to the United Hospital in the 19th century and most recently the Bath College of Art & Design until 2005.
The hotel's general manager is Martin Clubbe, who has headed the Macdonald Bath Spa hotel for the past five years, while chef Johann Lafer will oversee the hotel's restaurant - Johann Lafer at the Gainsborough. Originally from Austria and now based in Germany at Stromberg Castle, which houses the one-Michelin-starred restaurant Le Val d'Or, Lafer will create a menu featuring locally sourced English products combined with Asian flavours.
New York company Champalimaud Design has created the interior of the hotel, which is said "to pay homage to the building's fascinating history and Georgian characteristics in creating a modern interpretation of a classic design".
Meanwhile three students from Bath College of Art & Design have produced artwork to feature in the hotel's public spaces.
Named after Bath resident, painter Thomas Gainsborough, the property will be a member of sales and marketing consortium, Leading Hotels of the World.
Room rates of £285 per night will be discounted by 20% during the soft opening period of July. All revenue from the opening day will be donated to St John's Charity, which has supported Bath's disadvantaged residents since 1174.