Bridgestone tyres are abysmal, even so, I wouldn't expect wheelspin in 3rd unless the car has been remapped, and certainly not in 4th/5th/6th. Are you getting the traction control light coming on with this slippage in higher gears? If not, it's almost certainly your clutch slipping and not wheelspin as the revs pick up without the car going any faster. My R experienced clutch slip on a few occasions between 2600 and 3100rpm in 4th on the coldest days of Winter, with only 23k miles on the clock.
What puzzles me most is the possibility that someone replaced Bridgestone Potenzas with more Bridgestone Potenzas if your 2016 GTI has done more than 20k miles.
They are hard, and they take an age to warm up, but even when warm they're still hard, noisy and lacking grip/traction. I've ditched Bridgestones on 3 VWs - they ruin the car's capabilities.
If you've had the car 6 months, a garage is likely to rebuff any warranty claim for what they consider to be a wear and tear item (the clutch).
If it is the clutch, don't get a stock clutch replacement, they're crap. Get an updated clutch fitted by a reputable independent. My expectations of clutch life are 100k miles minimum if you don't ride the clutch at junctions (hard to do with an electronic handbrake).
