Guild Wars 2

How do you unlock outfits (not skins) that are not for purchase in the Gem Store?

You don’t until they are.

There are only a few (two that I can think of, off-hand) outfits that you get outside of the gem store. One was a reward for veteran players who’d bought the game before a specific time earlier this year. The other is a Halloween vendor item you could buy for candy corn cobs. All the others are gem store only, as far as I know.

Which brings us back to what I said