You'd have to remove the filler neck. Those have anti-siphon necks to prevent people from doing exactly what you're trying to do.
The 2006 GMC Sierra-1500 has 12 valves.
The 2006 GMC Sierra-2500HD has a V8 engine.
The 2006 GMC Sierra-2500HD has 16 valves.
The 2006 GMC Sierra-2500HD has overhead valves (OHV).
The 2006 GMC Sierra-1500 has overhead valves (OHV).
The 2006 GMC Sierra-2500HD has a 5-speed manual.
The 2006 GMC Sierra-2500HD is a gas-powered vehicle.
The 2006 GMC Sierra-1500 has a 5-speed manual.
The 2006 GMC Sierra-1500 is a gas-powered vehicle.
The 2006 GMC Sierra-1500 is 17 ft. 1.7 in. (205.7 in.) long.
The 2006 GMC Sierra-2500HD is 18 ft. 8.5 in. (224.5 in.) long.
The height of the 2006 GMC Sierra-2500HD is 6 ft. 4.2 in. (76.2 in.).