Small creatures have a +4 bonus to Hide, not a penalty (see Gyraspawn's Marsh-Kin ability). It looks like it should be Hide +28 (4 ranks [its only skill so I put all its points there] +4 Dex bonus, +4 size Bonus, +16 bonus from marsh kin).

I'm not sure how many ranks you gave the gyraskine.

Debby