We investigate the bond percolation model on transient weighted graphs ${G}$ induced by the excursion sets of the Gaussian free field on the corresponding metric graph. Under the sole assumption that its sign clusters do not percolate, we derive an extension of Lupu's formula for the two-point function at criticality. We then focus on the low-dimensional case $0< ν< \fracα{2}$, where $α$ governs the polynomial volume growth of $G$ and $ν$ the decay rate of the Green's function on $G$. In particular, this includes the benchmark case ${G}=\mathbb{Z}^3$, for which $α=3$ and $ν= α-2=1$. We prove under these assumptions that the critical one-arm probability decays with distance $R$ like $R^{-\fracν{2}}$, up to multiplicative constants.