In this paper, we prove that a space $X$ is a $g$-metrizable space if and only if $X$ is a weak-open, $\pi$ and $\sigma$-image of a semi-metric space, if and only if $X$ is a strong sequence-covering, quotient, $\pi$ and $mssc$-image of a semi-metric space, where “semi-metric” can not be replaced by “metric”.