Essentially, the GT6 gearbox is unique. It was driven by a clutch larger than that of the Spitfire, and the input shaft is larger and longer. Most importantly, ratios are much different than any Spitfire gearbox due to the far greater power and torque of the six-cylinder.
I would note that the early four-speed TR7 gearbox has ratios very similar to that of a typical GT6 gearbox, but I admit to knowing nothing about how well a TR7 gearbox might adapt to a GT6 engine (or, for that matter, a GT6 bellhousing and rear mount). Perhaps someone else here will know.
Vitesse 2L & Mk.II; GT6 and GT6 Mk.2/GT6+:
4th -- 1.0; 3rd -- 1.25; 2nd -- 1.78; 1st -- 2.65. Reverse: 3.10
TR7 ("Federal"):
4th -- 1.0; 3rd -- 1.26; 2nd -- 1.78; 1st -- 2.66. Reverse: 3.02