Hi & welcome!
Always interested in learning something new but as of today I don't know of any kind of plant that requires 2 individuals to produce flowers. Some kinds of plants are monoecious, meaning that individuals are either male or female, and one must have one of each for pollination to occur. Some of the male or female blooms of these might be so inconspicuous as to be overlooked. And, as Jean was saying, some plants require separate individuals for pollination to occur (self-sterile.)
But, a lone Gardenia is an unnecessary thing unless space is really tight. The only thing better than 1 Gardenia is more Gardenias. :+) You can snip a branch off, keep it moist in the ground (and a lawn chair over it for shade the first couple weeks can help,) and it's more likely than not to take root and grow a new shrub. I've had worthwhile success doing that. A couple I did 2 summers ago are blooming.
