High-energy positrons and bright γ-ray sources are of great importance both in fundamental research and for practical applications. However, collimated GeV electron–positron pair jets and γ-ray flashes are still rarely produced in the laboratory. Here, we demonstrate that by irradiating a near-critical-density plasma channel with two 10 PW-scale laser pulses, highly directional GeV electron–positron pairs and bright γ-ray beams can be efficiently generated. Three-dimensional particle-in-cell simulations show the formation of GeV positron jets with high density ($8\times {10}^{21}/{\text{cm}}^3$), attosecond duration (400 as), and a divergence angle of 14°. Additionally, ultrabright $[2\times {10}^{25}\text{photons}{s}^{?1}\text{m}{\text{m}}^{?2}\text{mra}{\text{d}}^{?2}{(0.1\%\text{bandwidth})}^{?1}]$ collimated attosecond (370 as) γ-ray flashes with a laser energy conversion efficiency of 5.6% are emitted. These features show the significant advantage of using a plasma channel as compared with a uniform plasma and thus open up new possibilities for a wide variety of applications.