MEDLANDS: R. de Torigny does not name her, and Marx does not try to ascertain her name; nor has it been found in charters. She is called Eustacie by Ramsay, presumably through confusion with her mother-in-law.
While MEDLANDS leaves her name unknown (as of June 2017), the Henry II projects says it is unknown but probably Mabel.
"Correction and Additions to CP" indicates that her name is Mabel. Richardson also follows this in Royal Ancestry (Vol. I p.17).
Family
m. William Gouet III, LORD or MONTMIRAIL[1] (b. ante 1080).[2]
Royal Ancestry by Douglas Richardson Vol. I page 17
MABEL OF ENGLAND, married before c. 1125 GUILLAUME GOUET (or GOET). They had one son, Guillaume Gouet. He married ISABELLE (or ELIZABETH OF BLOIS, Duchess of Apulia [see DONZY 4].
Royal Ancestry by Douglas Richardson Vol. II page 462
Genealogics: "She has also erroneously been named Eustacie (Eustacia) and Richilde"
Footnotes
↑ 2nd but 1st surv. s. and h. of William Gouet II, LORD of Montmirail and Chateau-du-Loir, and (jure matris) of Alluye and Brou, by his wife Eustache.
Henry II project says her name is unknown but probably Mabel. Genealogics says Mabel but "has also erroneously been named Eustacie and Richilde". MEDLANDS gives no name but mentions that her mother-in-law was Eustachie.