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I have heard it said that the GDP of the Soviet Union never got any where close to the US GDP. Since the USSR doesn't exist anymore, sources like the IMF won't display historical GDP for it. I am was wondering if there is any source listing both the countries GDP for comparison, or at least the Soviet GDP from say 1950 until its collapse in 1991. US GDP I can get from the IMF website.

Late Agnus Maddison site has excellent data:
Excel file:Statistics on World Population, GDP and Per Capita GDP, 1-2008 AD
1928-1990 USSR GDP as percentage of US GDP at Purchasing Power Parity:

Is there one at GDP nominal as well?

I doubt there is reliable data for nominal value of USSR GDP since it was a communist country however usually a country with middle income level will have nominal GDP about 50%-80% of PPP GDP.
For example here is nominal/PPP GDP ratio of some middle income countries today:
Russia 77%
Ukraine 50%
Poland 66%
Turkey 72%
Thailand 57%