It is not a hardy plant - you could risk putting it outside from first week of June through to mid/end August, but probably best treated as a rather fussy house or conservatory plant. If you want to repot, it needs acid soil, so ericaceous compost.

My book tells me that these are tender plants and can only be outside from late June until mid September. They should be in a pot 6"-8" in John Innes no2. and have a height and spread of 2-4 ft.
This is a very old book and plants and winters are different now, others may be able to give you more up-to-date info.

Gardenia is hardy to -6 deg. C, with gradual hardening off. Since it will be coming from or going to the warm indoors, it should be moved later in spring and earlier in fall than its hardiness would indicate. I would use an ericaceous mix and fertilizer for it, and give it plenty of sun indoors and out.

If you ever want more flowers, Guest, you need to keep the plant at not less than 16deg. C overnight, and at 21deg. C during the day.

Yep, those are the temperatures it needs to form and open flowerbuds.

Who needs a book when we have all you guys with great info? Thanks, you've helped me too.