You should know that Tretyakov Gallery was founded thanks to the backing of the Muscovite financier Pavel Tretyakov, an important collector of Russian art who supported many artists of his time. In 1892, he donated more than 2,000 artworks, his house and other buildings to the city of Moscow.
Fun fact: Pavel Tretyakov's brother Sergei was also a patron of the arts, collecting French and Dutch paintings, and he also donated his collection to the museum.