Quite a few retailers have signed up to participate, including both national chains like Target, Walgreens, Staples, American Eagle, and Toys“R”Us, as well as local SF stores like Blue Bottle Coffee, Palo Alto Toy & Sport, and Nob Hill Foods.
All the ordering and payment is handled via a Google website, saving the users the effort of navigating numerous partner websites. Delivery is handled by a courier service, not Google.
GSE is being described in the blogs as a competitor to Amazon Prime. Given that this is only a delivery service I don't see how that can be. Same day shipping is great but there's more to Amazon Prime than the free shipping. I don't see why this service would tempt be away from the free digital content that I can get from Amazon.
I think it would be better to say that GSE is intended to be a competitor to the same-day shipping service that Amazon offers in certain US cities. That isn't part of Amazon Prime, of course; it cost extra.