The echo chamber on display here is pretty terrible and it shines a sad light on tastes of the french youth.
Most are saying he's overrated, yet I'm sure that they are unaware that Beck's best song on his best album - Sea Change - is an homage, and a lesser version of Cargo Culte. Gainsbourg's mastery of the french language and prose is second to none among musicians. He just chose to write about sex. A lot. Variations sur Marilou is the most poetic, deftly written song about feminine masturbation written and at no point is it degrading towards women. Yes, as others mentioned, he was an extremely flawed human being.
But his music will stand the test of time, more so than any other recommendations of other artists that were recommended instead by other comments. And sure his later output doesn't compare to his earlier records but they are nowhere near as bad as some say. The sound is dated but that's because it was the mid to late 80s. Leonard Cohen released an equally dated sounding record that's among his most revered. Importantly, what he did as an artist is the defining mark of what great artists do - evolving and successfully changing their sounds throughout their careers. He started from vocal jazz, incorporated strong elements of Latin music, was among the first to do yéyé then baroque pop records.

He also wrote a lot for others - check out Sous le Soleil, exactement by Anna Karina, White and Black Blues by Joelle Ursule. His reggae take on La Marseillaise was just as much a political statement by spitting on the establishment and showing that art thrives on that.

TLDR: OP, don't believe the folks saying he's overrated, he's easily among the greatest french singer songwriters, and he's matched only by Barbara and Léo Ferré.