Alexander, if you do not have a country in a paricular world, you can simply sign up to one (the reference to 30 GC is because you'll have to pay 30 GC per 30 days for the registration).
In a world where you have a country, then you could pay 100 GC to get another one (but its far easier and cheaper to simply capture it).
To get your country, go to the 'simcountry portal', click on 'add country or enterprise', select your new world, then pick a country from the maps. There will be a confirmation pop-up that reminds you that you'll be paying 30 GC per 30 days for the registration, but you do not need to spend 100 GC to buy the country if you do not already have one on that world.