Gwyneth Paltrow says this Goop serum will have people asking for your skin care secrets. Getty Images; Stephane Cardinale - Corbis The A-list loves vitamin C . But Gwyneth Paltrow took her obsession with the skin care superhero ingredient a step further with Goop Beauty’s 20% Vitamin C+ Advanced Brightening Serum , a formula she recently redeveloped. “You really see results on your skin — next-level glow, softness, smoothness — when you use this serum,” she said in a recent Goop story . “People will ask you what you’re doing,” she added. Goop Beauty 20% Vitamin C+ Advanced Brightening Serum Goop Her two-part system includes a serum activator to ensure the formula stays shelf-stable for a full three months — addressing a common issue seen in other vitamin C-based products. Paltrow posted funny snaps with the bottle on Instagram. Instagram/@gwynethpaltrow Goop’s version is designed to brighten skin while improving firmness, tone, texture and hydration.…