GMC Sierra Owner's Manual page 28

Caution
To avoid damage to the tailgate, make
sure the area behind the tailgate is clear
before opening it.
If equipped with this feature, the vehicle
must be in P (Park).
Switches in the inner tailgate prevent the
primary tailgate from being opened when
the inner tailgate is not fully closed.
To open the primary tailgate:
Press twice quickly on the RKE
transmitter until the tailgate moves. See
Remote Keyless Entry (RKE) System
Operation (Key Access) 0 11 or
Remote Keyless Entry (RKE) System
Operation (Keyless Access) 0 13.
If equipped with Keyless Access, the
tailgate can be opened if the RKE
transmitter is within 1 m (3 ft).
Keys, Doors, and Windows
Press on the center stack.
Press the lower touch pad on the tailgate
handle after unlocking all doors. Use the
top of the tailgate to pull against if
assistance is required.
To close the primary tailgate, firmly push it
upward until it latches. Pull it back to be
sure it is latched securely.
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