Why is $()^{n}:\mathbf{G}_{m}\longrightarrow\mathbf{G}_{m}$ surjective in the $fppf$ topology?

Question

Let $X$ be a scheme. Why is $()^{n}:\mathbb{G}_{m}\longrightarrow\mathbf{G}_{m}$ surjective in the category $X_{fppf}$?

(Here $\mathbf{G}_{m}(X)=X\otimes_{\mathbb{Z}}\operatorname{Spec}(\mathbb{Z}[T,X]/(XT-1))$ is the multiplicative group scheme.)

In other words: Why does that map come from a faithfully flat morphism?

How does that relate to the case where one uses the Zariski topology instead of $fppf$?

A reference may be nice.