why is this not "gult"
Because it's definite. "det gule skjørtet" = "the yellow skirt", "et gult skjørt" = "a yellow skirt"
I'm also a bit confused by the sentence. Why it is not "gult skjørtet", gule would be plural, right? Also what "det" means, as a verb is missing.
Ok, it is explained here https://www.duolingo.com/skill/nb/Adjectives