I don't know what type of gown you are asking about, but I'm assuming you mean a formal gown. In general, I'd say keep it simple and one color throughout. The photos of myself that I hate most are when I'm wearing something with frills, at my size (5x) it looks ridiculous (on me), so I'd say stick to a classic style in a color that suits your coloring and make sure it fits well, if necessary make small alterations. If your dress has long sleeves, make sure they are the right length and fit well in width. A dress that fits well will always make you look slimmer.
If the gown has no sleeves and you'd feel more comfortable with your arms covered, a complementary pashmina will usually do the job, and these can be found at relatively low cost on Amazon. Just drape around your shoulders and go. If you are a pear shape, as most of us are, go with a slightly high waist and a trapeze shape. If you have a very large bust, don't pick a very low cut dress unless your bra is first class and you know it doesn't show. Can you go braless? Only you will know. I know I can't.
Wear a necklace that fits; it should sit around your neck, not too tight, and not touch the dress at all. Pearls suit everyone and go everywhere. An alternative is a brooch high on the shoulder, both shift focus to your face from your body.
If you want to show off a small waist, take a good look at the effect of a belt in the mirror. Many short fat people don't suit this look, not because they're fat but because they're short and the belt cuts them in two, whereas tall plus sizes can get away with it. It's not fair, but it's true. Tight waists and puffy skirts look pretty on the hanger, but when you're short there isn't as much skirt as there should be and it can all look horribly wrong. Abandon the princess look and go for the goddess look instead.
Most importantly, be comfortable and be sure to try the whole thing on well before the event you're going to so you can fix any problems.
My personal choice would be a high waisted, plain dress with no horizontal lines below the high waist, in deep moss green or black. To make it super formal, I'd add a beaded bag, appropriate makeup, my favorite necklace, and some sparkling earrings. Don't forget shoes, but make sure they've been worn in and are comfortable.
Remember, they should be looking at you, not the dress, no matter what size you are, your best accessory is a great, confident smile.