What is Gate Fidelity?

Introduction to Quantum Computing

"The average number of operations that can be performed within a qubit lifetime is a proxy for a more rigorous metric called gate fidelity."

Gate fidelity, then, is a rigorous means to define how well a gate operation works.  Errors can occur anywhere along the Bloch sphere and may manifest differently based upon where they start.  

The goal is to have a high gate fidelity.  The fidelity must reach the threshold to get a net improvement with error correction.