Does Gossick Suva really exist?

It doesn't exist, it's a country made up by the author, Ichiki Sakura.

In fact, as can be seen from the plot, it is mentioned in the play that several countries, led by Britain and Suva, became victorious countries in World War I because of the prophecy of fortune tellers, but if you look up World War I, you will find that there is no Suva at all.

Here are some references I found. In a word, Suva is a beautiful and mysterious country, but it does not exist. The name of Suva Kingdom is a homonym. In fact, the Loire River, the largest river in France, should be the Loire Valley. The name of the kingdom of Salem, which also exists in the Serengeti plain, is not homophonic, but only uses half of the former St. Margaret's College. In fact, France's four-star hotel Saint Margaret is also near the Suval River.

The description in the novels of Suva Kingdom is fictitious. Probably located between France, Switzerland and Italy. These can be seen in Wikipedia and other places. It should be that the author traveled in the Loire Valley in France and used some other nouns nearby. If you have time to search, it should all be related to it.