[technothrasher]

I'm not sure plurality is the distinguishing feature, since you have The United Kingdom, The Czech Republic, and The Gambia. But none of that really matters; if the country says it doesn't use the definite article, it seems pretty straightforward to not do so.

[azinman2]

Those are modified by kingdom and republic. There are many kingdoms and republics, which is why you identify it with “the”. That’s not what’s happening with Ukraine.

The Gambia is an edge case, as it’s requested by its government. From Wikipedia:

“The Gambia is one of a very small number of countries for which the definite article is commonly used in its English-language name, other than cases in which the name is plural (the Netherlands, the Philippines) or includes the form of government (the United Kingdom, the Czech Republic).“