When I hit the brakes my truck pulls to the right. What could be causing this?

There's three systems to inspect. More information, details, or observations would help narrow it down.

A sticking brake will not apply under light pedal pressure. That will make the truck pull toward the applying brake. A clue is often the truck will pull the other way for a little while after the brakes are released after hard braking.

The suspension system could allow a lower or upper ball joint to move under the stress of braking. That causes a change in alignment and the resulting pull. Worn control arm bushing are fairly common on GM vehicles but the clue there is you'll usually hear and / or feel a clunking noise when you hit the brakes.

Worn parts in the steering system will allow one wheel to turn out during braking. The steering wheel will change position a little when that happens. An experienced alignment mechanic can tell the difference between a steering wheel that shifts position while the truck continues to go straight vs. A truck that pulls to one side and you have to physically turn the steering wheel yourself to KEEP the truck going straight.

In any case, the best approach is to have the brakes, steering, and suspension systems inspected at a tire and alignment shop. The people there know how to inspect each part properly and what to look for.