GMC stands for General Motors Truck Co., and SLE stands for special luxury edition. General Motors manufactures different vehicles with different features. SLE is a package of standard equipment found on General Motors vehicles. "GMC SLE" is therefore a General Motors vehicle with standard equipment as per the company's standards.
GMC SLE vehicles are further divided into the SLE-1 package and SLE-2 package. Although the latter has more features than the former, they both have enough conveniences. The SLE-2 is more expensive than the SLE-1 due to the extra conveniences it has, such as a premium sound system and a leather-wrapped steering wheels.