There are three possible ways to flat tow this vehicle without destroying your transmission:
You can put a manual transmission vehicle into neutral (this doesn't work for automatic transmissions, and I'm not sure that GM offered the option for that model year), or you can put the transfer case into neutral in the case of a four wheel drive vehicle (manual transfer cases and the four button electric transfer cases offered by GM have a neutral position; the three button electric transfer cases do not), or, if neither of these conditions applies to your vehicle, you'll have to remove the driveshaft. Just trying to flat tow it without preparation is a great way to wreck a transmission.
The 2011 GMC Sierra-1500 has a V6 engine.
The 2011 GMC Sierra-1500 has 12 valves.
The 2011 GMC Sierra-1500 has overhead valves (OHV).
The 2011 GMC Sierra-1500 is a gas-powered vehicle.
The 2011 GMC Sierra-1500 has a 4-speed automatic.
The 2011 GMC Sierra-1500-Hybrid has a V8 engine.
The 2011 GMC Sierra-1500-Hybrid has 16 valves.
The height of the 2011 GMC Sierra-1500 is 6 ft. 1.6 in. (73.6 in.).
The 2011 GMC Sierra-1500 is 17 ft. 1.6 in. (205.6 in.) long.
The 2011 GMC Sierra-1500 runs on regular unleaded.
The 2011 GMC Sierra-1500-Hybrid has overhead valves (OHV).
The 2011 GMC Sierra-1500's turning circle is 39.7 ft..