I was looking at some google N-grams and noticed that the ratio between usage for gern and gerne has a strange bump in the late 1940s. In particular, gerne gained in usage but then returned to the expected curve in the late 1950s. (N-gram link.) It's not just the relative rate, you can see the same feature in the percentage data (link).
My initial guess is that it has to do with changes in publishing after WWII, or books being destroyed, or something along those lines (rather than actual change in usage).
I'm not a langauge scholar and I'm barely a German speaker, so any thoughts or insights would be greatly appreciated. (Probably this is another case of a question that should be a PhD thesis.)