Hpw do you get a preset with the texure on GIMP? Idk how to make layers slightly- transparent and all... I'm sorry for the inconvienice of my confusion.
I'm not all that familiar with Gimp (As I use Photoshop) but if Gimp works in a similar way, then open up the texture and drag it onto your preset's texture. Then move the fur tex around until it is in place. Set the layer mode of the fur to 'multiply', and you should be able to see the preset tex underneath!
I hope that made sense, I'm not very good at explaining things ^^;