With the Met Gala 2022 theme of "Gilded Glamour" comes the strict rule for white tie. White tie was the standard formalwear attire in the 19th century, and since the theme will focus on that time period, it stands to reason that the dress code will pay homage to that historic era as well.
White tie is the most formal dress code. For example, you can wear a tuxedo for black tie, but for white tie, it requires a tailcoat. If you're opting for a dress, than a ballgown — accessorised by white gloves, a tiara, or ornate jewellery — would fit the bill.
Some mainstream examples of the white tie dress code are period dramas (think a "Bridgerton" ball).