Questions & Answers about Sé natural y haz ese gesto.
Why does sé have an accent mark here?
In this sentence, sé is the irregular affirmative tú (informal "you") command form of ser (to be). The accent mark tells it apart from the pronoun se (used in reflexive verbs). It looks exactly the same as yo sé (I know) from the verb saber, so you rely on context!
What verb does haz come from?
Haz is the irregular affirmative tú command for hacer (to do / to make). Normally, informal commands use the present tense él/ella form of the verb, but hacer has this special, short form.
Why is it ese gesto and not eso gesto?
Why does this sentence use the command form of ser (sé) instead of estar for "be natural"?
We use ser here because being "natural" is treated as a core characteristic or personality trait. Even if you are just asking someone to act that way temporarily for a photo, Spanish uses ser to describe the fundamental quality of the behavior.