I seem to recall that the name “Guillaume” was pronounced differently in Norman French.
It was actually closer, if memory serves, to the English “William.” I seem to remember “Guillaume” was pronounced “wee-OM” and not “gee-OM” as in modern French.
I would love to find some confirmation of this. Wikipedia seems to support this by claiming that the name is from an Old Norman name “Willame” but has no information on the correct 13th century Norman pronunciation. I assume “Willame” is a much earlier form of the name though from when, I’m not sure as William the Conqueror’s name on the Bayeaux Tapestry is spelled “Willelm” or “Willem” and my name was spelled alternately as “William” or “Guillaume” (as is the case with Le Marechal).
Edit: Apparently “Guillaume” was pronounced “gwee-YOM” or “Gwee-ZHOM” in Old Norman. I was close!