The General Staff Building forms the long curving south side of Palace Square, opposite the Winter Palace, and is now the home of the Hermitage's collection of 19th- and 20th-century European art — including the museum's celebrated French Impressionist, Post-Impressionist and early modernist holdings. The interior has been comprehensively reworked over the past two decades into a sequence of bright, white-walled galleries that contrast sharply with the gilded splendour of the main Hermitage across the square.
This is where you will find Matisse's Dance and Music, the room of Picassos, Gauguin's Tahitian paintings, and a strong room of Russian avant-garde including Malevich and Kandinsky. Most short Hermitage visits skip the General Staff Building entirely, which means it is far less crowded than the main museum. Allow at least two hours; allow a half-day if you love the Impressionists.