Andrew,
Unfortunately this issue was very common on '14 and '15 models of the Sierra, Silverado and Full size '15 and up Suv's
No it is not powertrain. Powertrain would be Transmission, Engine (internal) etc.
Another item to look out for is the Ac Condenser (in the very front seen thru grille) that develops stress cracks and leaks refrigerant leaving you with blowing hot air.
The Vacuum pump is also another known failure item. You are covered 6 years or 72K miles on that if you notice any slow speed hard braking effort in the pedal.
I take it that your Silverado did not come as a CPO certified pre-owned?
If not, nothing other than powertrain is covered since you are out of the standard warranty period.
There is a 100K corrosion warranty I believe.
Aside from that welcome to the site and club. You will find Plenty of back and current info here on your truck.
Despite these issues as all automakers have them, your Silverado should be a worthy steed!