The gid keyword (generator id) is used to identify what part of Snort generates the event when a particular rule fires. For example gid 1 is associated with the rules subsystem and various gids over 100 are designated for specific preprocessors and the decoder. See etc/generators in the source tree for the current generator ids in use. Note that the gid keyword is optional and if it is not specified in a rule, it will default to 1 and the rule will be part of the general rule subsystem. To avoid potential conflict with gids defined in Snort (that for some reason aren't noted it etc/generators), it is recommended that values starting at 1,000,000 be used. For general rule writing, it is not recommended that the gid keyword be used. This option should be used with the sid keyword. (See section 3.4.4)
The file etc/gen-msg.map contains contains more information on preprocessor and decoder gids.