I must confess that my mother tongue is not English ... So I went to Google to ask questions about names, and must also be said that even a few names "common" does not sound so familiar to those who do not usually listen in movies in English. Anyway I found several references, so I used to, although I would not classify as a popular name, but just look and you'll find several references, the fact that it is a name of flower, as a woman's name, helps of course, and seems to me an old name too ...
I could have used Rose, but found it so common ...
Some refs:
http://www.sheknows.com/parenting/articles/845005/50-pretty-girl-names

http://www.parenting.com/baby-names/girls/Gardenia

http://www.kidsnames.com/girlsg/gardenia.html