i guess they are in a semi-shaded site? you would want to plant a companion with similar light requirements and any color pairs well with the white gardenia flowers.
maybe just some summer annuals to attract interest after the gardenia flowers are gone. they only are in mass bloom for about a month.
if you just want to stick with green, maidenhair fern would offer a different texture and shade of green to the dark, shiny foliage of the gardenia. caladiums might be nice if the spot has late afternoon shade.

During the winter it is in full shade because it is under an overhang on the North side of the house. By Feb/March the border gets sun for most of the day. I've been considering:

Dietes (Fortnight Lily)
Agapanthus (Lily-Of-The-Nile)
Rhaphiolepis indica (Indian Hawthorn)
Nandina domestica (Heavenly Bamboo)
dwarf Myrtus communis
Hebe
dwarf thornless Carissa macrocarpa (Natal Plum)
Roses underplanted with something...Roses would need to few thorns though, maybe 'Pink Gruss an Aachen'. Has anyone else tried pairing roses with gardenias??

The climate is very mild so I'm sure there are many more exotic selections to consider. I'm afraid the ferns, which are gorgeous, would burn out here.

I have citrus trees(orange and lemon), Laurus nobilis 'Saratoga' (Sweet Bay), Magnolia grandiflora 'Majestic Beauty', iris, hot pink bouganvillea and carolina jasmine near the backyard gate, and a property line border of Coleonema pulchrum (Pink Breath of Heaven) on the other side of the driveway and two olive trees in the front yard near the gardenias.

I've even been tossing around the idea of lavender (I love fragrant plants), although I'm not sure how well lavender would tolerate the winter shade. Any thoughts on lavender in the shade in winter? There is a drip irrigation, so I can keep them dry in the winter. I know lavender doesn't like being cold and wet. If I thought I could keep a Michelia or Osmanthus fragrans under 3 or so feet, I would put it on my list. Anyone know of a dwarf Osmanthus fragrans???

I'm looking for something that will be evergreen or mostly evergreen, fairly tidy and soft-ish (no thorns) as it is beside the sidewalk. Ideally, fragrant and/or showy too.

I like the blue and white combo of gardenia and hydrangea.

In the same bed I have a border of white narcissus with a blue primrose that look stunning. I think a mass planting of blue primrose sprinkled with clumps of white narcissus would look stunning... ooh perhaps with one of the blue lepidote rhododendrons- like 'bluenose' or 'Blue Baron'. I think somewhere in there a good white fragrant rose would fit in as well (an alba perhaps?).

I also have monkshood and Va. Bluebells and Daphne in the same bed.

Will Iris grow where you live?
There are dwarf Iris that might look nice, they don't need alot of water and some varities bloom in spring and fall.
I have a gardenia planted in the front of my house too, but I don't have anything right next to it.
I have same sun exposure that you do.
I have dahlias (dwarf varities) planted about 3 feet in front of it.
The dahlias appreciate the afternoon shade.
Looks nice.