she was invited by a for-profit private company that poses as a fundraiser for the Met. She was apparently donated a ticket as well as as donated a seat at a paid table (see below for market rate). i.e., purchased access or campaign donation in kind? She may or may not have purchased the dress, but instead of just walking in, she devoted several minutes to modeling for the designer.
“How much is a ticket to the Met Gala 2021?
Various prices have been listed for a Met Gala ticket, but according to the Evening Standard it cost $35,000 for a single ticket in 2019 and $200-300,000 for a table at the event.”
(no event in 2020)
https://wear-next.com/news/met-gala-2021-date-theme-time-tickets-happening-cancelled/
Unless she bought her own $35K ticket it seems quite illegal.