I am interested in this also as I like the GXW-56 watches.
However I would say that whilst Casio will honour a 1 year manufacturers warranty, you will not get a 2 yr local shop warranty like H Samuel jewellers offer.
Importing one from Japan will likely mean you have to pay an import duty fee too so perhaps you could look up info on this. Other than that, people buy and sell G-Shocks from all over the globe so as long as your seller offers a return policy for international buyers who may have a faulty watch, then you should be fine.
That's what I reckon anyway.