why were the germans called gerrys in ww2?

Answer 1

cause it sounds a lot like germans, and it's easier?

Answer 2

Because 'Gerry' is short for 'German'. The English love to shorten names - hence Paul Gascoigne became 'Gazza', the politician Michael Heseltine 'Hezza'. It's all very childish and goes back to when English males were educated at public schools (i.e. private schools!) and gave each other nicknames - it can be heard at it's best on the BBC radio test match commentaries which are all done by public school boys.