This plant has been growing normally for a while but I woke up this morning to discover brown patches and some shriveled leaves. I had previously sprayed it down with fungicide and then with insect-repellent, so what now?
I'd like to have seen a photo of this plant when you first got it, its hard to believe it was healthy only three weeks ago.
Stop feeding immediately, you've been overfeeding it anyway. I'd also say don't ever spray a plant with fungicide or pesticide unless you know it is infected or infested, when you should identify the problem and use the correct remedy, diluted correctly. This plant may not recover, but keep it somewhere that has a minimum temperature of 60 deg F, with an even temperature, not near a heat source such as a radiator (if its winter where you are). It needs a brightly lit spot, but not direct sunlight, and as it is currently in very poor health, switch to watering with lime free water (not tapwater). You can boil water and let it cool and water with that, or use rainwater, although tepid rather than cold water is best. Mist what's left of the leaves frequently, do not allow the plant to dry out completely, keeping the compost moist, but not wet or waterlogged - good drainage is essential, and don't leave it sitting in water in any outer pot or tray. IF it recovers, then you can start feeding, but this should only be done in spring/early summer, and only at recommended rates and frequency, and preferably using only one type of feed at a time.
There is no point in feeding a plant at the wrong time of year, not least because you're wasting your money. In terms of plant health, most plants respond to growing day length and temperature, even indoors, by starting to grow in spring and early summer, and this is the time, for most, when feeding is appropriate, particularly in pots. If you use too much fertiliser, then the pot is full of concentrated feed - this may burn the roots, or the plant, when it wants a drink, is obliged to also uptake the feed, whether it wants to or not. In open ground, this is not such a problem, because rain dilutes and the plant is surrounded by soil which its roots aren't occupying anyway. This is definitely a case of 'less is more'. Use only what's necessary, when necessary.
With regard to lack of growth, it might not be lack of nutrients that's the issue - the amount of light a plant receives makes a big difference, along with other environmental conditions such as heat sources, cold draughts, how good the growing medium was in the first place, size of pot, humidity or lack of it, along with the obvious problems with infestation or infection. If you really think your plants hardly grow during spring and summer, take a photo of them in winter, then another at the end of summer, and compare the two. There should be growth, no matter how minor - it just might not be as fast as you'd like, many plants are slow or medium growers, and giving too much food will not ameliorate this. It is also normal for plants to stop growing after end of July (in the northern hemisphere) - some houseplants may produce a small amount of growth over winter, but the majority of growing is done in spring and summer.
Just for information's sake, Gardenia is a difficult houseplant, bit of a diva - its usually purchased in flower and then doesn't flower again, it needs high, even temperatures to produce flowers, and is extremely finicky about its conditions.