Removing gear is damn expensive, does it just eat the gear when I sacrifice a champion, say, when I want to upgrade my Galek to 5 star, and I use my 4 star skullsworn as food, does it eat all my purple gear I have on him?

Yes. Any champion used to be sacrified, equipped with gear, will have the gear be removed with it.

so I advice you to take a look at what gear u have on the champion, pay attention to the rank, main stat (for bottom row gear), and sub stats, to determine the value of the gear