I think it would be "de Grece", I've never seen it as Gres.
I have his book Crown Jewels and it's an excellent book, his research of his subject is meticulous. Michael wrote a blog for many years, it included not only Royal histories, but he wrote on many other subjects as well, it was an interesting read.
I remember Michael said that when his mother died, the Count and Countess of Paris literally took him in as of their own along with their 11 children. Countess Isabelle supposedly stated, "We have 11 children, what's one more to love."