it doesn't look to be in bad shape to me. they will lose some older leaves as they grow. it could be that the container is too small for that shrub size if it is drying out in two days. can you plant it in the ground? (hawaii?)
they prefer afternoon shade in hot climates like alabama.
also sounds like a lot of fertilizer for that container.

Sorry, I gave it two teaspoons, not tablespoons, of fertilizer. Unfortunately I can't plant it, I'm in an apartment. It's getting about 4-5 hrs of direct afternoon sun.

I guess I'll just keep an eye on it. I'm just nervous about yellowing leaves because I lost a smaller gardenia to rapid leaf yellowing and dropping off. Whole plant was bald in a 3 days.

Lagoonlyrics,
Gardenias roots rot very fast if they are too wet.
It is actually better to have them too dry, than too wet.
Growing a gardenia in a pot is challenging.
The drainage is the problem.
You would be surprised that the gardenia really isn't dry down by the roots when you think it is.
Go to a discount store and buy a cheap moisture meter for plants. Costs about 3.00.
THen put the end down the pot next to the roots to check for moisture before you water it again.
When a gardenia dies from root rot it turns brown starting at the bottom of the plant and it works it way up.
THe leaves nearest to the main stem turn first.
Yellow leaves aren't alarming, but brown is bad.
Try to shade it a little in the heat of the afternnon sun also.
They really don't like that.
Stop putting stuff on it.
Just buy the meter and learn when to water it.
Gardenias don't like to eat when they are stressed anyway.
Like people.
GOod Luck to you.