Redditor atrubetskoy created this map of nations as their names are used in the Russian language. In Russian, nouns are either feminine, masculine, or neuter, and if it is a plural, the noun loses its gender, except in a few cases of mainly old forms of the word. There are no articles in Russian, either. He also posted notes to further explain some usage and exceptions:
• Соединённые Штаты (United States) is plural, but Америка (America), which is feminine, is used most often outside of formal context.
• Перу (Peru) and Чили (Chile) are most often neuter when talking about the political entity, but feminine when talking about the physical country.
• Somalia seems to be in anarchy even in the Russian language. Сомали (Somalia) can be seen variously as masculine, feminine or neuter.
• There is a similar ambiguity with Тонга (Tonga) and other Pacific Islands; generally they are masculine by default.
• As someone has pointed out, Нидерланды (Netherlands) is the more formal name in Russian, which is plural. Голландия (Holland) is not incorrect, however, unlike in English; it is feminine.
• Turkmenistan and Kyrgyzstan are masculine by their official names, which are rarely used favor of the "-ia" feminine versions.
• For the UK, Великобритания (Great Britain, fem.) is the standard. Соединённое королевство (United Kingdom, neut.) is seldom used, and Англия (England, fem.) is inaccurate.
• The Dem. Rep. Congo and Rep. Congo are marked as feminine, which is correct when using the long names. "Конго" (Congo), however, is neuter.
• Израиль (Israel) is colored incorrectly; it is masculine. This was an accident; I was not implying that it is part of Палестина (Palestine, fem.).
People are asking how the gender is determined.
• Historical countries in Russian end in -ия (-ia) and are thus feminine, with a few exceptions (China, Lebanon, Algeria).
• Younger or smaller countries have names that are simply transliterated. If they happen to end in -a or -ia, they will generally be feminine (except Tonga). Otherwise, they are mostly masculine.
• A few countries are neuter, and these generally end in -o (Morocco, Monaco, San Marino, etc.). Chile and Peru are exceptions because they end in a vowel, which makes it sound awkward in masculine; this means they are in a limbo between neuter and feminine, as explained previously.
• Because Russian does not have articles, the genders of some countries are much less clear-cut. Сомали́ (Somalia) sounds awkward in any of the three genders. Because gender can only be memorized, unsure writers avoid this altogether by saying "the nation of Tonga" or "islands of Tonga", changing the grammatical subject.
Although the singular gender isn't relevant in Russian most of the time, a user pointed out that it does matter in the genitive case.
• Соединённые Штаты (United States), Объединённые Арабские Эмираты (United Arab Emirates), Коморы (Comoros), Малдивы (Maldives), Сейшельские Острова (Seychelles), Соломоновы Острова (Solomon Islands), Маршалловы Острова (Marshall Islands) are masculine.
• Филипины (Philippines) are feminine.
• Сент-Винсент и Гренадины (St. Vincent), Тринидад и Тобаго (Trinidad and Tobago), Сан-Томе и Принсипи (São Tomé and Principe) and Фиджи (Fiji) are not declined, and therefore ambiguous.