G and GT just refer to the shape of the glass, G is tapered with a blobby top and GT is straight sided. The works are identical,
Have you checked the valve heaters are alight?
I've no experience of Fender kit, but in valve amps the most likely one to blow is the rectifier 5Y3, followed by the 6V6 output valve. Rarely is it a 6J7

The heater of the 5Y3 has a different supply (5v)to the 6- series valves (6.3v), so if none are alight (a red glow down the centre, not always easy to spot if the glass is greyed on the inside) look elsewhere for the problem!