Stephen Ashmore said he had “put a lot of work” into Unique Fitness on Ryton Street, which was due to open next week.
But the building was destroyed in a fire that it was believed was started deliberately in the early hours of Monday morning (February 13).
Stephen said: “A lot of work had gone into that gym.
“It was a community effort with many electricians and decorators doing the work for free. A lot of people wanted it open. I’m not sure what to do now.”
The site is being cleared today as the remainder of the fire-ravaged building is knocked down.
Polie are appealing to anyone who saw anything suspicious in the area at the time.
They are particularly keen to speak to two slim built men seen wearing dark clothing and carrying what looked like a holdall at around 2.55am on Newcastle Street.
If you have any information, call 101 quoting incident 61 of 13 February.
You can also contact Crimestoppers anonymously on 0800 555 111.