The rule for when to prune any flowering shrub is to do it immediately after it's done flowering so it has plenty of time to develop flower buds for next year. (Never right before flowering, of course!)
So prune your gardenia after it's done blooming. There's really no need to prune a gardenia bush unless it's getting too large for the spot. Just cut out any obviously dead or damaged wood. And then, if desired, cut back a branch here and there with a hand shears or long-handled loppers there to reduce the overall size.Don't shear the bush with a hedge clippers or power trimmer. It will distort its natural shape and shred the attracive leaves.