What does (W), (GS) and (PS) mean?

(W) stands for women's sizing.

Women's sizing ranges from UK 2.5 - UK 9.5.

(GS) stands for junior (gradeschool) sizing.

Junior sizing ranges from UK 3 - UK 6 (UK 6 = EU 39/40).

(PS) stands for kids (preschool) sizing.

Kids sizing ranges from UK 10 kids - UK 2.5.

Mens sizing will often not be mentioned in the listing (I don't make the rules)

Men's sizing ranges from UK 3-14 although it's very hard to find a mens UK 3-5.5 as very few are produced.