they are very similar but have some differences. here is a section about the differences from Yemoos.com
Ginger Beer vs Water Kefir
Both are symbiotic colonies of bacteria and yeast and perform similar
functions of fermenting sugary-water, but that is about where the similarities
end. Just like milk kefir and kombucha are cultures of the same principle yet
totally different, the same can be said for ginger beer and water kefir.
Their shape, translucency, diet preferences, fermenting time and even mineral
preferences differ. The resulting taste is also notably different. Ginger beer
having a more sour bite - some even feel it's like a mix of a tart kombucha and
sweet water kefir. The grains never get nearly as large as water kefir's greatest
size, and do not form the same angular, clear grain - instead they are like a
murky brown puffed rice. They can range in size from tiny little tapioca globs,
to extra big pea-size blobs. GBP also grows much more slowly. It's easy to strain
water kefir in a colander, but you might lose your ginger beer doing the same!
Sometimes, you may even get a kombucha-like scoby on the top at the end of
a ginger beer ferment (depending on time, ingredients and temperature)!
Try them both, and see for yourself!
from a little google searching water kefir grains seems to be composed of different microbes than GBP and also seems to be more diverse in its composition from culture to culture, similar to kombucha mushrooms.