I've never heard of Vartan, and I was never able to get into Hardy. I can't really compare their careers to France Gall's. However, FG's career spanned twenty years or so, and her style changed much over that time period. This is another reason I think her career is similar to SR's: the sheer amount of change in her music and image.
I don't understand what you mean by "plain pop artist". I mean, Ringo herself has always been trying to be "innovative and in vogue at the same time" as well. Sure, FG didn't write her own repertoire, but from what I've been able to gather, she selected her writers and shaped her image/style - which is an art in itself. In my opinion, stuff by FG is better than most material by performing songwriters now.
And, as an added bonus, here's another controversial song by Gall. This is called "La Petite", which is a song about a young girl falling in love with a friend of her father's. The creepy part is that the guy she is singing the duet with (Maurice Biraud) actually was a friend of her father's. This song was written by Serge Gainsbourg, who also wrote "Les Sucettes". English lyrics are in the YouTube description.