We visited GUM while strolling around Red Square. The facility is enormous, with wonderful glass ceilings stretching the full length. The 'department store' is set up more like a shopping mall, with many small stores throughout. It's a great place to stop over the use the toilet while visiting Red Square, as the facilities are clean and free.
The shops are very nice although not on the cheap side. There is also a large upscale grocery store on the ground floor, which had wonderful pastries and cakes.
While we visited, GUM had many balloons to celebrate a birthday, and also had a display of racing cars on the ground floor, which amazed our child.