Learn Danish: Gutterne skal ud i aften. - The guys are going out tonight.

In Danish, it is very common to drop the main verb of motion (like gå or tage) when a modal verb like skal is paired with a directional adverb like ud. Saying skal ud perfectly conveys "are going out" all on its own.

Why use ud instead of ude here?

You use ud because it shows direction or movement (going out). If you used ude, which indicates being stationary, the sentence would imply the guys are already out on the town, rather than heading out.

Does gutterne have a different feel than just saying drengene?

Yes, gutterne translates best to "the guys" or "the lads" and is highly colloquial, used for a group of male friends. While drengene ("the boys") is also used this way, it can sometimes just mean literal children, whereas gutterne specifically captures the social vibe of guys hanging out.

Why does skal mean "are going" here instead of "must"?

While skal often means "must" or "have to", it is also the standard way to express a firm future plan or arrangement. In this context, skal ud i aften means it is a set plan that they are going out tonight, rather than a strict obligation.