The following extract from The New a York Times explains the look that “flatland gear”appears to refer to:
Orange Shasta: Flashbacks to Doc and Shasta’s life together at the beach show Ms. Waterston in a “uniform” of bikini bottoms, loose-fitting cotton shirts and no shoes, “simple, pretty, sexy,” Mr. Bridges said. “And you’re not sure if you’re just seeing her in Doc’s mind’s eye or if it’s real. I thought it would be a fond memory if she was always walking around like that.”

But when she first appears in a relatively prim orange crocheted mini-dress and a neatly combed hair-do, the narrator describes her as “looking just like she swore she would never look.” This “flatland” gear, as Mr. Pynchon described it in the novel, means she is from the Los Angeles basin or Hollywood, Mr. Bridges said, anything but the beach. But how to do that and keep Shasta’s sex appeal? “A hard shoe with a heel was a good starting point,” he said. “A bra was also something she never would have worn before. And then in the scene she talks about the kind of kinky way Mickey liked her to wear short skirts. So we made her presentable, but still sexy.”