The CMS Global Calorimeter Trigger (GCT) is the device within the Level-1 CMS calorimeter trigger system which is assigned the tasks of finding and sorting forward, central and tau-jet candidates, sorting isolated and non-isolated electron candidates and reading out all of the calorimeter trigger data. The GCT system has been installed and commissioned in the CMS underground cavern. A sophisticated software package has been developed for controlling and configuring the GCT hardware and monitoring the GCT status. Over the past two years the GCT system has undergone detailed testing and its performance is well understood. The GCT design provides for buffers at the inputs to the GCT which have been used to inject energy depositions corresponding to electrons and jets and test the GCT functionality by comparing the GCT output with that of simulation. Monte Carlo events simulating the decay of Higgs particles and other processes have been used to validate the performance of the GCT. The GCT has also been commissioned with the other components of the Level-1 trigger chain in cosmicray muon runs. Results from these studies are presented.