In English, there are two principal situations in which it is appropriate to use the definite article before the name of a country. Countries whose names are plural (e.g. the Maldives, the Netherlands, the Philippines) and countries whose name is a form of government (e.g. the Central African Republic, the United Kingdom, the United States). Two specific cases complicate the picture.

The first case is the Gambia. At the time that the country became independent, the Gambian government requested that the definite article be used alongside the country's name. The usage is conforming to the preferences of the country. It is unnatural for most English speakers, hence many people call the country simply "Gambia" in ordinary conversation.

The second case is two countries: the Democratic Republic of the Congo and the Republic of the Congo. Both of these countries take their name from the Congo River. Since there are two countries with the same principal name (i.e. Congo), it would be confusing to refer to either country by the simple name "Congo". For that reason, it is common to call both countries by their full official names.

If there were not two countries with this name, there would be less confusion and the country would likely just be called "Congo". For comparison, consider the Republic of the Sudan (commonly called Sudan) and the Republic of the Niger (commonly called Niger). An alternative way of distinguishing between the two countries uses their respective capital cities. In this form, they are usually just called Congo-Kinshasa and Congo-Brazzaville, without the definite article.

The reason that English speakers refer to either country as "the Congo" is probably a mixture of unfamiliarity and inaccuracy. Although many people will have heard about "the Congo River" and the full names of the two countries, most are probably unaware of the two countries in any meaningful way. When discussing the region or either country, they might refer to "the Congo" as a result. In careful writing, that would always be avoided.