The gathering technique is called ruching
You've got a very good start here, and you look quite lovely! One thing that would use some more attention is the tailoring/fit on the bodice. It's a bit baggy in several areas, leading to a wrinkled, unfitted appearance, whereas Belle's dress is very fitted through her torso. Here's how to fix it (you'll need a second person to help you in the back):
Put on the undergarments you intend to wear with the dress, and put the dress on inside-out, so you have easy access to the seams. Get some pins handy and start with the two seams down the front of the bodice. At about an inch or so above your natural waistline, take hold of one seam and pin the two bodice pieces together, maybe about 1/4 to 1/2 inch from your original seam (put the pin through the two layers of the bodice that are the garment itself, not the seam allowance, of course). Do the exact same thing on the matching seam on the matching seam on the other side of the front of the bodice. Then repeat this above and below the first pin you put in on both seams. Keep adding pins until you've pinned in a full new seam line. You may find that in some areas (across the bust, for example), no additional tailoring is necessary, and all you have to do there is smoothly transition from your existing seam into the new seam you're pinning in place. Do this entire process again on the seams underneath your arms, and then on to the next seams in the back of the dress (here's where you'll need assistance). You'll probably need to go back and re-pin seams several times over, adjusting how much you are taking each seam in, to get a perfect fit. This will effectively reduce the size of the bodice by fitting it more accurately to your physical shape.
Now, you likely have about 6 or so seams around the bodice. DO NOT try to make your bodice fit by altering only the front two seams. ALL seams will need to be pinned, sometimes adjusted multiple times, until every seam has been taken in. It is important that each seam is treated in pairs - underarm left and underarm right, bodice front left and bodice front right, back left, and back right - so that the same amount of fabric is reduced identically for each seam, otherwise, your garment will end up lop-sided (if the left seam is taken in more than the matching right seam, for example). The seams down the torso should run more or less vertical (within reason; there's curves over the bustline, for example), so keep this in mind when you're pinning down the seams - if you try to take in too much, these seams will start slanting and running all akimbo.
Once you have pinned all your seams, congratulations, you now have a bodice that is more accurately tailored to YOU, rather than some unknown pattern model's dimensions. If you're satisfied with the fit, and everything is symmetrical, now re-sew all of your seams down the lines where you pinned. You will likely be much happier with the sleek appearance of the bodice now!
A few caveats: it is highly likely that you'll end up taking in so much of the bodice around your waist, torso, and hips that it will be necessary for you to remove the skirt. Sorry; it sucks, but typically, bodice tailoring is done before skirt attachment to prevent such a difficulty. You may be able to get away with just pulling out the skirt-to-bodice attachment stitches right around each of the bodice seams you will be altering, and then re-attaching the skirt in those areas so you don't have to take the skirt off entirely. Be careful with this shortcut, though, as you don't inadvertantly want to damage the skirt in any of the pinning, resewing, and trimming that will be necessary for tailoring the bodice.
Finally, a bodice of this style, to REALLY have that nice sleek appearance, aught to have boning flanking each seam around the torso to provide some structure and support. However, to do this properly, it requires boning channels, which are best put in a sturdy lining layer, and not in the more delicate outer layer, where the boning channel seams will be seen. If you want to line the dress, it is possible to add such a thing after finishing the dress, but it will be a little tricky, and it will require you to create an exact duplicate of the bodice AFTER you've already tailored the bodice, which can be a bit of a pain to finagle. It is possible to engineer boning channels into the seam allowance of a bodice after it has been completed, and not have the channels show on the outside, but it's not really the most ideal set-up. Adding a liner and boning would be a challenge, but on the other hand, that sort of attention to detail, structure, and construction is generally something that pushes a decent cosplay up into the next tier of quality. If you ever decide to do a second version of the Belle dress, I'd definitely suggest reading up on garment lining and boning techniques.