Gowns have a reputation for being the one purchase you only use twice and regret both times, bought in a hurry for an occasion that dictated the whole decision. I think that reputation is earned by bad shopping rather than by the garment itself, and a good one is a different proposition entirely.
The trick with a gown is not to treat it as something separate from everything else you own. The ones that get worn more than once are the ones that pair easily with things already in the house: a plain coat you can throw over it without ruining the line, shoes in a colour you already have in two other pairs, a bag small enough to carry and dull enough not to compete. Before you buy, look past the picture on the model and check the things that decide whether it survives an evening. Feel the weight of the fabric between your fingers rather than trusting how it drapes on a hanger. Look at the zip, not just whether it closes but whether it's sewn in straight and backed properly, because a crooked zip will announce itself the first time you sit down. Check the lining goes the whole way, not just to the knee with hope for the rest. Check any boning or structure sits where your body actually curves rather than where a smaller sample size assumed it would. A cheap gown often looks fine photographed still and falls apart the moment you move, walk, sit, or reach for a drink, while a good one is cut with the understanding that a body in a gown is rarely standing perfectly upright with its arms by its sides.
A gown that's made well will tell you so after a few wears rather than one. The seams stay flat, the fabric doesn't go shiny at the hips where you've sat in it, the hem doesn't start dropping after a single dry clean. Have it cleaned properly rather than steamed in a bathroom and hope, and store it on a proper padded hanger rather than folded in a bag at the bottom of a wardrobe, because creases in heavier fabric can take as long to drop out as they took to form. If you've none at all, start with the plainest cut you can find in the best fabric your budget allows, a simple column or a clean bias cut, rather than anything embellished in a fabric that can't carry the embellishment. Decoration hides a cheap cut for about one wear and then shows its hand.
Reach for it when the occasion is formal enough that nothing shorter will do, and buy it as though you intend to wear it again, because the good ones always give you the chance to.