  For the critical level-set of the Gaussian free field on the metric graph of
$\mathbb Z^d$, we consider the one-arm probability $\theta_d(N)$, i.e., the
probability that the boundary of a box of side length $2N$ is connected to the
center. We prove that $\theta_d(N)$ is $O(N^{-\frac{d}{2}+1})$ for $3\le d\le
5$, and is $N^{-2+o(1)}$ for $d=6$. Our upper bounds match the lower bounds in
a previous work by Ding and Wirth up to a constant factor for $3\le d\le 5$,
and match the exponent therein for $d=6$. Combined with our previous result
that $\theta_d(N) \asymp N^{-2}$ for $d>6$, this seems to present the first
percolation model whose one-arm probabilities are essentially completely
understood in all dimensions. In particular, these results fully confirm
Werner's conjectures (2021) on the one-arm exponents:
  \begin{equation*}
  \text{(1) for}\ 3\le d<d_c=6,\ \theta_d(N)=N^{-\frac{d}{2}+o(1)};\ \text{(2)
for}\ d>d_c,\ \theta_d(N)=N^{-2+o(1)}.
  \end{equation*}
  Prior to our work, Drewitz, Pr\'evost and Rodriguez obtained upper bounds for
$d\in \{3, 4\}$, which are very sharp although lose some diverging factors. In
the same work, they conjectured that $\theta_{d_c}(N) = N^{-2+o(1)}$, which is
now established. In addition, in a recent concurrent work, Drewitz, Pr\'evost
and Rodriguez independently obtained the up-to-constant upper bound for $d=3$.
