Hundreds of US companies imported items from Japan, with their own company name on them (GW Co is unlikely to be the name of the Japanese manufacturer). About all you can do is estimate its age by the style of the design and whether it's handpainted or not. There are some books out on Made in Japan items, available at Barnes & Noble and other large booksellers. Those might contain more info for you.
As for values: vintage and antique items really don't have a 'set' value - that depends on where, how, when and to who it's sold. Even an insurance appraisal will give a different evaluation than an appraisal for resale value.
If your item is shaped like this one, it's likely to have been a 'blank' available for those who handpainted porcelain items:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&
item=2047350906
Here's an item where at least part (the metal part) is marked 'GW Co', yet it's a Harker Pottery item.
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&
item=2050876923