The GT gives you the windscreen, more laid back bars, plusher seating, heated grips, and the backrest as well. I'm 6' and have the pegs in the back position, which is fairly close to the mid-position of the R model. Sit on/ride both and see what you think.
I test drove both. At 5’9”, 32” inseam, the GT controls (+/- 1”) were too far forward, like a typical forward control HD. I was taken with the red R but Triumph really made it a pita to achieve a comfortable ride. The original Rocket was a perfectly comfortable ride. Why did they go and F it up?
Before the 3GT I had a 2018 Fat Bob with a similar feet forward position as the 3R. I got used to it, but never really liked it a 100%.
When I bought the 3GT the foot controls were already replaced with the 3R ones. I was lucky to find one that was a cancelled order so didn't have to pay for the conversion.
Asked the dealer and was told that it is easier (cheaper) to move the foot controls than the handlebar, and as someone already mentioned, you get the added benefit of the 3GT "comfort" additions too.