Yeah, I would also assume that
Gwendolyn is the most commonly used and thus the most classic spelling, but I was interested to see that on this unofficial list of 'most popular names in England and Wales in 1890',
Gwendoline is ranked #112 and
Gwendolen #198, whereas
Gwendolyn doesn't appear on it at all:
http://www.britishbabynames.com/blog/2012/09/top-200-most-popular-names-in-england-and-wales-in-1890.html Maybe the
Gwendolyn spelling was/is more popular in the US?