"Kamu harus gugup, ya?" Is this also correct?
That's not right. If you say "kamu harus gugup", it's like you're ordering the person you're talking to to be nervous.
In this kind of phrases "You must be/have to be/have got to be ..., right?" it's better to translate it to "Kamu pasti ..., ya?"
Kamu pasti gugup, tidak?
It's not accepted. But why not?