GMC Sierra Classic 2007 Owner's Manual page 150

NEUTRAL: Use this position when you start or
idle your engine.
REVERSE (R): To back up, first press down the
clutch pedal. Wait for the vehicle to stop moving
and then, shift into REVERSE (R). Let up on
the clutch pedal slowly while pressing the
accelerator pedal.
Notice: Shifting to REVERSE (R) while your
vehicle is moving forward could damage
the transmission. The repairs would not be
covered by your warranty. Shift to
REVERSE (R) only after your vehicle is
stopped.
Also, use REVERSE (R), along with the parking
brake, when turning off your engine and parking
your vehicle.
150
Shift Speeds
CAUTION:
{
If you skip a gear when you downshift,
you could lose control of your vehicle.
You could injure yourself or others. Do
not shift down more than one gear at a
time when you downshift.
Do not allow the engine Revolutions Per
Minute (RPM) to overspeed and enter the
tachometer red lines. Engine overspeed occurs
when downshifting, or descending a grade, at too
high a vehicle speed. When going down steep
grades, always select a gear at least one position
lower than you would use on climbing a grade.
This will allow the vehicle's engine to provide some
braking and help to slow the vehicle as it goes
down the grade. Monitor your vehicle speed,
acceleration and engine RPM while applying the
brakes as you go down the grade to ensure
you have complete control over the vehicle.