  Despite rapid progress in understanding gapped topological states, much less
is known about gapless topological phases of matter, especially in strongly
correlated electrons. In this work we discuss a large class of robust gapless
quantum spin liquids in frustrated magnets made of half-integer spins, which
are described by gapless fermionic spinons coupled to dynamical $Z_2$ gauge
fields. Requiring $U(1)$ spin conservation, time reversal and certain space
group symmetries, we show that certain spinon symmetry fractionalization class
necessarily leads to a gapless spectrum. These gapless excitations are stable
against any perturbations, as long as the required symmetries are preserved.
Applying these gapless criteria to spin one-half systems on square, triangular
and kagome lattices, we show that all gapped symmetric $Z_2$ spin liquids in
Abrikosov-fermion representation can also be realized in Schwinger-boson
representation. This leads to 64 gapped $Z_2$ spin liquids on square lattice,
and 8 gapped states on both kagome and triangular lattices.
