Gimp isn't Photoshop, plug-ins for PS don't work out of the box in Gimp. There is a thing called PSPI that lets you use some PS plugins on Gimp (no personal experience, it's for Windows), but for something as frequent as the white balance, there are scripts, the best one IMHO being Luca De Alpharo's whitebalance script.
If you are trying to correct a wrong color temperature setting on you camera, the same person has a colortemp script.
If you are trying to correct a wrong color temperature setting on you camera, the same person has a colortemp script.