If he only replaced the gasket, then the cams would stay in the head, and thus the shims would not be touched. If he sent it to the machine shop to be checked for warpage and cracks, then the cams and springs were probably removed.
The shims sit on top of the valve stems, and are under each cam lobe.
They must be adjusted when ever they are taken out, and when a valve job is done. Most people never adjust them after the initial settings are done, even when they get a little noisy from wear.
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