What he said! Retro suntour 7 speed cassettes will not fit shimano hubs as the splines are different. However some newer suntour bits will. Sounds lke your gt probably has retro suntour bits with their non- shinano compatible splines. Assuming the gear levers and derailleurs on your gt are suntour then your best bet would be try and repair the hub or replace with a retro suntour one in good condition. Decent photo of the bike taken from the side might help with further advice.
You may get away with a shimano hub and cassette with suntour gears, but as the spacing across suntour cassettes was different to shimano, then suntour shifters and derailleurs work less well with shimano cassettes.