We propose topological one-way fibers enabled by the recently discovered Weyl points in a double- gyroid (DG) photonic crystal. By annihilating two Weyl points by supercell modulation in a mag- netic DG, we obtain the photonic analogue of the 3D quantum Hall phase with a non-zero first Chern number (C1). When the modulation becomes helixes, one-way fiber modes develop along the winding axis, with the number of modes determined by the spatial frequency of the helix. These single-polarization single-mode and multi-mode one-way fibers, having nearly identical group and phase velocities, are topologically-protected by the second Chern number (C2) in the 4D parame- ter space of the 3D wavevectors plus the winding angle of the helixes. They are readily realizable at microwave frequencies with magnetic materials. This work suggests a unique way to utilize higher-dimensional topological physics without resorting to artificial dimensions.