GBP is not the same as water kefir grains, though they do look a bit similar and you can make a knockoff of ginger beer using water kefir grains (and you will find people selling water kefir grains as GBP. You may see these listed as tibicos or sugary kefir....you have to know what you are looking for and what you want is a plant from DSMZ Strain 2472(genetic testing so the true GBP can be identified).
The ginger beer plant and water kefir are visibly quite different. The ginger beer grains are smoother, smaller, and more opaque than the water kefir grains. To me, GBP can have a sandy quality when the plants is small, but as the plant gets older they become larger but they are still smooth, and they don't grow in the leaps and bounds that water kefir grains do.
You can visit www.yemoos.com, or www.happyherbalist.com, or www.gingerbeerplant.net for a great overview (and these sites usually have GBP for sale).