  Spin-gapless semimetals (SGSMs), which generate 100\% spin polarization, are
viewed as promising semi-half-metals in spintronics with high speed and low
consumption. We propose and characterize a new $\mathbb{Z_{\mathrm{2}}}$ class
of topological nodal line (TNL) in SGSMs. The proposed TNLSGSMs are protected
by space-time inversion symmetry or glide mirror symmetry with two-dimensional
(2D) fully spin-polarized nearly flat surface states. Based on first-principles
calculations and effective model analysis, a series of high-quality materials
with $\textit{R}\overline{3}\textit{c}$ and $\textit{R}{3}\textit{c}$ space
groups are predicted to realize such TNLSGSMs (chainlike). The 2D fully
spin-polarized nearly flat surface states may provide a route to achieving
equal spin pairing topological superconductivity as well as topological
catalysts.
