GMC 2018 Terrain Owner's Manual page 191

190 Driving and Operating
transmission to slow the vehicle.
The system will automatically
command downshifts to reduce
vehicle speed. The normal shift
pattern will return once the vehicle
is on a low grade or when the
accelerator pedal is pressed.
While in the Electronic Range
Select (ERS) Mode, grade braking
is deactivated, allowing the driver to
select a range and limiting the
highest gear available. Grade
braking is available for normal
driving and in Tow/Haul Mode. See
Manual Mode 0 180.