First of all, relax. People who gift bonsai to others with no knowledge of how to grow them deserve whatever happens :0). Giving any plant (short of a philodendron or something) to anyone is risky, let alone something requiring specialized care. Anyhow, the yellow leaves are from too much water, ok, but not the end of the world, and the fact plenty of water was left over just shows that the tree knew to only take in so much (rather than turning to soup). The blooms may certainly have been ready to drop anyway - many flowering plants only keep them a day or so at a time. Not watering for "several days" may be the way to go, but not necessarily, so compensation for overwatering is not always the answer depending on the plant, what it's planted in, where it's located in terms of light, etc. etc., which is not to say you shouldn't wait until at least the top layer of soil is dry, and maybe even more (it's impossible to judge from here). What you do need to find is a place in a very sunny location (indoors) for it, but also an airy, and even somewhat cool place, as they prefer it to hothouse conditions. Once leaves have actually died (vs just turning a bit yellowish) then take them off the tree and new ones should grow, even if not for weeks or even months, as it's now early winter (and the hours of light it gets will tell it that spring's months away). Don't sit the pot in water ever again, and water only from above, slowly, so it doesn't run off the sides, til you're sure all of the mix has had a good drink. DO have a wide tray of stones and water under the pot for humidity (don't spray it) but the stones should always keep the pot above the water, so roots don't rot. Do see if you can find a local bonsai club as that's the best place to learn because you can see how things are done, and question what terms mean vs guessing, etc. etc. Right now your mission is to keep it alive over the winter and get an idea of how often (or not) to water, plus how much light it likes. Things like pruning and shaping (both roots and everything else) can wait indefinitely. And you know what? If you do lose it (don't be impatient to decide that's happened though at any time) then you've joined the illustrious club of newbies here. You didn't ask to be thrown in the deep end (and gardenias are a lot deeper than, e.g. a Ficus would have been) and all you can do is your best.