I would not use it in a terrarium.
Here is a link below to a commercial site on general care of your type of bonsai.
I recommend checking out more sites on general gardenia care for additional insights.

Here is a link that might be useful: commercial page on care of gardenia bonsai

Thanks for the link terrestrial man...I'll continue to seek advice on gardenias in general.

Any particular reason why you wouldn't recommend using it in a terrarium?

Here is more info on growing.
While the plant can handle it up to 80F I think most are
happy with cool moist roots (like camellias) and
excellent drainage.
Notice the leaves. I do not think humidity is an issue
with this type of plant.

Here is a link that might be useful: how to grow gardenias

Hi
I grow several types of tropical trees which are maintained dwarf as well as potted. All but two have been moved out of terrariums due to size.
You might be able to grow the gardenia in a terraium but certainly not the species of choice lol. Gardenias are tough for lots of people including me lol. They have all kinds of problems. Interesting that I can grow gardenia in the yard but will kill it in a pot every time.
There are many other species that would be better choices for a terrarium than gardenia.. but who knows might work but I'd bet it will be tough gary

Would a mango sprout do alright if it were potted, and placed in a terrarium?

Mango? Give it a try.
You may have to prune the roots some as it grows and
perhaps reshape its trunk. Got any bonsai type wire?

I have a bonsai ficus in mine and it is doing well. It's the centerpiece of the whole tank.