This is a known issue with almost all models of the MBP, my work provides us with these models and luckily enough - We're allowed to use them for personal as well when off the clock. I've had the last three models, this issue is rampant with these laptops. Your best bet is to underclock and undervolt your GPU with afterburner or some similar software.

Mobile GPUs are designed to be run at 70c or under, this is their thermal throttling point unlike desktop GPUs where the thermal throttle is at 81c. It is possible to overwrite the throttle but I do not advise it.