How are you pronouncing them to get
Gil as a nickname?
Objectively, I prefer
Guillaume, but I think they are both very tied to their respective languages of origin, and I would be very surprised to meet a
Guillermo who wasn't Spanish or a
Guillaume who wasn't French. I'd honestly much rather see a
William, or even a
Willem.
Ottilie
http://www.behindthename.com/pnl/116467