That's not chanteur/provocateur Gainsbourg, but rather Eric Elmosnino, who will play him in the upcoming Serge Gainsbourg : vie héroïque. The film is being directed by French comic book artist Joann Sfar, best known in France for his Chat du Rabbin series. Francophones out there can watch this video in which Sfar was interviewed by le Figaro about the project. Caveat emptor: it's a pain to load.
Sfar has spent the past year preparing for the film, drawing storyboards, costumes, and characters. He says it will be a fable that closely follows the life of Gainsbourg, and claims he chose actor Eric Elmosnino because he most resembles his drawings of the legendary singer. Shooting starts in September.
Here's Gainsbarre singing "Les Sucettes" with France Gall, who became a star performing songs he wrote for her and who allegedly was shocked to learn that the lyrics he wrote contained sexy double entendres.

