We'll start with the 3.0-liter diesel inline-six and 10-speed automatic, as both are more broadly available for 2020. The diesel adds the second-lowest Elevation trim level, meaning it's available in every trim except the base Sierra. The diesel is also now an option for the CarbonPro Edition AT4 and Denali, so you aren't limited to the 6.2-liter V8 for those trims. The 10-speed automatic is finally available on the 5.3-liter V8, but it's restricted to four-wheel-drive versions of the SLT, AT4 and Denali trims – lower trims retain the eight-speed automatic. The 10-speed transmission continues to be standard with the 6.2-liter V8 and 3.0-liter diesel.
GMC also added new towing-related features for 2020. It has a newly-optional rear-view camera system that can show what's behind the truck's trailer, and it can project that where the trailer is on the main display, effectively making the trailer disappear. This feature was introduced in the Sierra HD. GMC also added a feature to its trailer app that lets you store and share details on your trailer setup so that it's easier for someone else to hook it up. You can even monitor and activate functions on compatible trailers with the app.
There are a number of other more minor updates. Sierras can now be optioned with adaptive cruise control. The Elevation trim is available with the Crew Cab configuration. The AT4 CarbonPro Edition gets a few extra black accents compared with the 2019 version. Pricing and availability of the 2020 Sierra haven't been announced yet, though.