This is Umi City's big Supermarket.
In case you didn't know, a supermarket is not a place to find super-stuff to save one's day. It's more some sort of a big grocery store.
The Supermarket has doors that open automatically, and lots of aisles filled with all kinds of food, supplies, and housewares. When no one is using the P.A., the store plays the music-box typed music from Team Umizoomi's theme. The cash register is an electronic device (with a friendly self-checkout lady voice) that calculates the amount for whatever someone bought and tells them to thank the team for shopping at Umi City Supermarket after paid.
Gary: He's the only worker we know about and can be only seen in City of Lost Penguins. He called Team Umizoomi to tell them that penguins are in the Supermarket.