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The 'gc' in the configuration directives here stand for garbage collection which means cleaning up the session files after a certain period of inactivity, as jatar_k mentioned. It does not necessarily mean that sessions will be cleaned up after the set period of inactivity. It does mean that there is a certain percentage of probability that it will be cleaned up after that period of inactivity. In the example directives shown there is a 100% chance that sessions will be cleaned up every 30 seconds.
The link provided describes the configuration directives in greater detail. Also, here are a couple of other good threads that may be of interest:
session not timing out [webmasterworld.com]
Session is timing out - how do I increase cookie time? [webmasterworld.com]
joined:Nov 12, 2005