The English "William" also starts with a "G" in Italian-- and the Gallic versions of the names have came back to English as new names-- William-- drops the Wi (there is no W in gallic) and becomes Liam, John ( J too, is missing became Sean (from the french Jean-) or Ian -- James became Shamus.

I always thought Richard to Rich- to Rick to Dick.. And Margaret to Marge --Margy Meg- Meggy- Peggy.

And some might just be childish corruptions--Judith-- to Judy-- to DeeDee-- because a older brother "heard" Judy as DeeDee. That's the same way my cousin Tansy got named-- Frances to Francie to Tansy..