Provable by Gentzen Proof System iff Negation has Closed Tableau/Formula

Theorem
Let $\mathscr G$ be instance $1$ of a Gentzen proof system. Let $\mathbf A$ be a WFF of propositional logic.

Then $\mathbf A$ is a $\mathscr G$-theorem :


 * $\neg \mathbf A$ has a closed semantic tableau

where $\neg \mathbf A$ is the negation of $\mathbf A$.

Proof
This is a specific instance of Provable by Gentzen Proof System iff Negation has Closed Tableau: Set of Formulas.