The GIA does not review or authenticate jewelry. Someone who has taken classes from GIA might, though. There are various studies that one can take at GIA to earn credentials, but that doesn't mean that someone who has these credentials is the GIA.
I hope that makes sense. I'm just not sure it was clear the way you were talking about it.
Edit...just in case it's not clear, the GIA does grade individual diamonds and gemstones.
Thanks for your input.
Beyond GIA certificates on diamonds I've bought previously, I must admit that my knowledge is fairly thin.
That said, it does appear that eBay have a contract with GIA, and the items are sent to a dedicated GIA facility;
Authenticity Guarantee for Jewellery | eBay.co.uk
Shop fine jewellery verified by the experts at GIA, the Gemological Institute of America.
pages.ebay.co.uk
And the international/US site;