Newly planted plants need a thorough watering, not a little, whether or not it has rained. The soil around the roots needs to settle around them and provide ample moisture to keep them going until the plant taps into the ground moisture. Straight after planting it's not much different from a cut flower in a vase. Give it a real soaking and then see how it does.

First rule. Never amend the soil in the planting hole--use the soil you dug out.

Thanks @floraluk2 I believe you're right as I think back to my watering. I've been so timid about overwatering it, I didn't think about it I could have not given enough water to the plant. There's an azalea I put next to it that appears to be doing fine so I didn't even think about that. Thanks so much for your insight!

@laceyvail thank you! I wish I didn't have to amend it, but here we have really heavy clay soil which isn't the best for their roots, if I solely use it. I learned the hard way with some rhodies I did that way last year. 😞

I think it's OK to amend the soil. You just need to blend it with the surrounding soil. What you don't want is effectively planting in a bucket of compost.

I always dig out a generous hole, then add the dirt that falls off when teasing the roots of the plant. Mix well, using the hole as a mixing bucket, and plant. I only ever add compost when there is a large rock or root that comes out of the ground to replace the missing volume.

OP, gardenias can be a little dramatic. Just keep it well-hydrated and I think it'll be fine. I think all mine did a similar thing when I planted them. Just ignore the tantrum and it'll be over with shortly.