"A white skirt"
"Gwyn" is an adjective that has a masculine and feminine form: "Gwyn" is the masculine and Gwen is the feminine. "Gwyn" will not mutate directly after a noun because it will never come after a feminine noun. Gwen always mutates when it comes after a noun because it only comes after a feminine noun. Because "Sgert" is feminine you use the feminine form so "Sgert"+"Gwen"="Sgert wen".