Not much. The phone gets heat if you use continuously for more than 6 hours, which is quite normal in all big battery smartphones.
Yes, Moto G5 does have Gorilla Glass Protection.
Moto G5 3GB RAM dimensions are 144.3 x 73 x 9.5 mm.
Yes, you can update this phone. However, this phone comes with latest Android 7.0 Nougat out of the box. So, you may not require to update the phone immediately.