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The operation that verifies a gas piping system does not leak is a leak check. This process involves applying a suitable leak detection method to verify that no gas is escaping from the piping system. This can be done using various methods such as a soap solution that bubbles at the site of a leak, electronic leak detectors, or other appropriate means to ensure the integrity of the gas piping.

While a pressure test can also indicate if there are leaks in the system, it primarily checks the system's integrity under specific test conditions rather than verifying operational safety. A purge operation is used to remove air and other contaminants before gas is introduced into the system, and a visual inspection helps to identify obvious issues, but neither guarantees that the system is leak-free.

Therefore, the leak check is the most direct and crucial operation for ensuring the safety and reliability of a gas piping system.