Mass deaths of Soviet prisoners in Germany during World War II are still being covered up, and the situation should be changed, the German president added.
"When it comes to the mass consciousness, these people are completely forgotten. It is unfair towards them," he said.
Gauck said he was going to pay tribute to Soviet prisoners of war next week, when Germany would celebrate the 70th anniversary of the liberation from Nazism.
The president also claimed Germany may consider compensation to Greece for damages caused by Nazi occupation during World War II. Earlier, Berlin repeatedly stated that it would not consider the issue of reparations.
WWII lasted from 1939 to 1945 and involved 61 countries and over 80 percent of the world's population. Some 70 million people are believed to have perished in the conflict.