They be gross.

listeneverThe girl replies: They be gross.Is this what the girl actually said?

Yes, that's right.

Does she mean 'they are gross'?

Well, I've got another question.
At 1:25, the reporter said something like "_________ right?" Followed by a girl saying "It was funny."
What did the reporter say in the blank?

listeneverDoes she mean 'they are gross'?

They behave in a "gross" manner.

listeneverWhat did the reporter say in the blank?

Hard to tell. The laugh track drowns out the reporter's voice.

AlpheccaStarsThey behave in a "gross" manner.

Thanks. Is 'they be gross' natural or is it what only kids can say?

AlpheccaStarsHard to tell. The laugh track drowns out the reporter's voice.

I hear something like 'up here'. Does that make sense to you at all?

listenever Is 'they be gross' natural

It is natural in African-American Vernacular English, and other dialects.
It is considered an error in standard English.

AlpheccaStarsIt is considered an error in standard English.

I don't think so here. The child is asked 'What do they do?' and she answers naturally 'They be gross' (= 'they act gross'). It is similar to e.g. 'What do cowards do when confronted?' — 'They be silent' or 'They be afraid'.

Mister Micawber'They be silent' or 'They be afraid'.

I wouldn't want an English learner to get into this pattern.
As stand-alone sentences, they be on a slippery slope to the first ring of grammatical purgatory.