If its a 305, toward the rear left of the air filter cover
You add fluid the same place you check the fluid level. If you have a newer GMC the transmission may be a sealed unit with no place the check the level or add fluid.
NO. You must drain it out to the correct level. To much fluid will damage the transmission before it pushes the fluid out of the vent tube.
It is either on the back left in the engine department. Or you have a full plug on the transmission and its full when it spills overs.
The 2011 GMC Sierra-1500 has a 4-speed automatic.
The 2003 GMC Sierra-1500 has a 5-speed manual.
The 2007 GMC Sierra-1500 has a 4-speed automatic.
The 2012 GMC Sierra-1500 has a 4-speed automatic.
The 2006 GMC Sierra-1500 has a 5-speed manual.
The 2004 GMC Sierra-1500 has a 5-speed manual.
The 2008 GMC Sierra-1500 has a 4-speed automatic.
The 2001 GMC Sierra-1500 has a 5-speed manual.