Antony Beevor, the author of The Second World War, described Prof Gerhardt's methodology as "ludicrous".
"It's almost impossible to come up with figures, but I think to say there were hundreds of thousands is a great exaggeration," he said.
"If she's doing it on the basis of illegitimate children that's ludicrous. There was a huge amount of voluntary sex. There were vast numbers of cases of genuine fraternisation. Many young women were hanging around outside the gates of American camps."
The most notorious instances of rape by Western Allied forces were by French troops during the sacking of Stuttgart.
Of the Allies, British troops appear to have been responsible for the fewest rapes.
"Not because of any morality or respect for woman, but because the NCOs wouldn't allow the soldiers to go off on their own," Prof Beevor said.
He added that Soviet archives had confirmed that around two million German women were raped by Soviet soldiers, although Prof Gerdhardt put the figure at 500,000.