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Create a new version of the grandcampaign.eug file, rename it to something else, open it in a word processor, change on of the country abbreviations (i.e. POL for Poland-Lithuania, HAB for Austria) to the abbreviation of the country you want to play. When you select that new scenario, the country whose abbreviation you entered will be available in place of the country you replaced.
 
Originally posted by Hallsten
how do I change the countries you can play in the GC if I'd like to play e.g. china?
just tell me where in what file. :p

Take a peak at my FAQ, there's an 'editing the game' section.