For the GDR, see:

1. "What You Should Know About the Wall"
2. "The Berlin Wall: Another Cold War Myth" by William Blum (author of Killing Hope)

For the Soviet Union, this is a thing you hear about many countries (Cuba and the DPRK for example). Since it's not accompanied by any data, you can simply say that people also immigrated to the USSR (or wherever else). This is equally valid. They don't know how many people immigrated to and from the Soviet Union, but they have an image cultivated by years of propaganda: so ask them. They'll be forced to tell you that they don't know. Then you will be done with it.