Glitter Gloss is a girly store that Carly, Kayla, Sam, and apparently Gibby often go to, along with Build-a-Bra. Freddie hates this store, along with Build-A-Bra, calling them "girly". Based on its name, it is most likely a cosmetics store that prides itself on the sale of scintillating lip gloss.
It was first mentioned in Season 1 during iFence and not mentioned again until Season 5 in iCan't Take it. Gibby got a text alert that said they were having a "big sale at Glitter Gloss" in order to get the girls out of the apartment, and have Freddie's mother try to break Sam and Freddie up.
Spencer: Bring me some glitter..