GMC 2015 Sierra Owner's Manual page 300

For example, when M5 is selected,
1 (First) through 5 (Fifth) gears are
automatically shifted by the
transmission, but 6 (Sixth) cannot
be used until the plus/minus button
on the shift lever is used to change
to the range.
In vehicles with gasoline engines,
when the shift lever is moved from
D (Drive) to M (Manual Mode), a
downshift may occur. The gear that
the transmission is operating in
when the shift lever is moved from
D (Drive) to M (Manual Mode)
determines if a downshift occurs.
See the following chart.
6 Speed Automatic Transmission
Gear before shifting from D
(Drive) to M (Manual Mode)
Range after shifting from D
(Drive) to M (Manual Mode)
8 Speed Automatic Transmission
Gear before
shifting from D
8th
(Drive) to M
(Manual Mode)
Range after
shifting from
D (Drive) to
M6
M (Manual Mode)
Tow/Haul not
engaged
Range after
shifting from
D (Drive) to
M6
M (Manual Mode)
Tow/Haul
engaged
Driving and Operating
6th 5th 4th
M4 M4 M3
7th 6th 5th 4th
M6 M5 M4 M3
M5 M4 M3 M3
9-37
3rd 2nd 1st
M2 M2 M1
3rd 2nd 1st
M3 M2 M1
M3 M2 M1