Those are 100% fine temps, in fact your GPU is running a lot cooler than I would expect it to. My 980M hits 85 on occasion in The Witcher and the CPU has nailed 90 a few times while it normally never goes above 78-80. 🙂
(Though tbh, one of the most discussed topics are temperatures so I'm surprised you haven't come across this information while digging around.)
I don't recommend changing the paste - it's not a completely simple thing and you can end up with an unserviceable brick.
I do recommend using Intel XTU to change the core voltage offset down as far as it can go, I'd start at -75mV, there is no risk except maybe getting a blue screen, which is your test measure. Also I think you should be able to use Notebook Fan Control, NitroX has provided a fan profile for the G751JY, which I can only assume works for the JT. I've seen a couple users using it successfully on the JT.