  Gapped systems with glide symmetry can be characterized by a Z_2 topological
invariant. We study the magnetic photonic crystal with a gap between the second
and third lowest bands, which is characterized by the nontrivial glide-Z_2
topological invariant that can be determined by symmetry-based indicators. We
show that under the space group No. 230 (Ia-3d), the topological invariant is
equal to half of the number of photonic bands below the gap, and therefore, the
band gap between the second and third lowest bands is always topologically
nontrivial, and to realize the topological phase, we need to open a gap for the
Dirac point at the P point by breaking time-reversal symmetry. With staggered
magnetization, the photonic bands are gapped, and the photonic crystal becomes
topological, whereas with uniform magnetization, a gap does not open, which can
be attributed to the minimal band connectivity exceeding two in this case. By
introducing the notion of Wyckoff positions, we show how the topological
characteristics are determined from the structure of the photonic crystals.
