Who is Gael? We don't know. We know she has a mother, and that her father's name is Rhince (though he doesn't seem to be a member of the crew, as the book Rhince was). It would appear that Gael and the man beside her are both from the peasant class on the Lone Islands. Theories suggest she's a Narnian who tags along to find her mother in the slave trades, or possibly a released slave trying to catch up with her mother.
The dress is a dusty pinkish purple color, possibly a linen or cotton weave. It's long, just touching the tops of her matching purple boots.
The neck appears to have a V-notch cut into it—we can't tell if it buttons or ties there. The skirt is attached with a seam just at the waist. Looks like it laces up the back in this picture. At first it almost looks the skirt has a darker purple trim at the bottom, but the skirt is simply wet around the hem.
The sleeves are somewhat reminiscent of Lucy's Blue Narnia dress from the first film. These sleeves, though, are split, open almost to the elbow, with rounded corners.