A roof rack can be considered as a some kind of construction set. You can increase the ability of your GMC to carry different things which have been impossible to transport before. For instance, there are horizontal rails on a vehicle's roof, but adding a couple of crossbars with a bike rack would allow for carrying a bike or skis. Accessories are necessary to join different parts together, for repair and for other purposes. Anyway, a rack system includes a lot of components. Source them all from a single brand if you build your individual rack structure. In this case, accessories will match each other perfectly, and the whole structure will look better.
Our store recommends Thule rack systems and accessories for installation, as they have a practical, elaborate structure, excellent quality, and perfect fit to each other. Here you will find a wide selection of Thule accessories for your GMC: cross foot packs for installation of crossbars, air screens for improving the aerodynamic features and style, load stops to fix your stuff in a safe and stable position, end caps to cover bar ends, and other products. All of them are made to last and will serve for years to come.