Kori wasn't even a Gecko before really, Fine-Tooner is just simplifying for the sake of clarity.

When someone is 'tooned' by TFT, they have a more plastic look and feel to them, it varies from victim to victim, but usually they end up being made-up of a mix of the usual: Polyester, PVC, Latex, Rubber, and even Clay/Putty on occasion. On the internal side of things, the fluids you can find in them include: Ink, Paint, Gelatin, Pop, Chocolate, Cheeze Wiz, ect. Again, dependent on who's been tooned.

I'm sure in Kori-I mean, Giddy's case, they're full of Gusher's Jam and Club Soda. To feel all bubbly all day long!