Q: Engine is not turning over; clicking noise from starter

The symptoms you describe on your GMC Sierra lead me to believe that the issue lies with the battery, starter or the wiring that joins these two components together. It is also possible to have issues with all three. You also mentioned the battery and starter tested OK. While there are a variety of ways to test these components, testers have been known to provide false positives. With that being said it is also possible that your battery is not providing enough Cold Cranking Amps (CCA) to the starter so it can turn the engine over. Another possibility is that the starter is in fact getting enough CCA, but the solenoid in the starter has failed, preventing your vehicle from starting. In some cases faulty wiring can also be to blame, whether it be a bad connection or a damaged power wire. To be 100% certain it may be time to enlist the assistance of a certified mechanic. They will be able to determine exactly why your vehicle is not starting and perform the necessary repairs to get your vehicle on the road again.