  High-energy positrons and bright {\gamma}-ray sources are unique both for
fundamental research and practical applications. However, GeV electron-positron
pair jets and {\gamma}-ray flashes are still hardly produced in laboratories.
Here we demonstrate that, by irradiating two 10 PW-scale laser pulses onto a
near-critical density plasma channel, highly-directional GeV electron-positron
pairs and bright {\gamma}-ray beams can be efficiently generated.
Three-dimensional particle-in-cell simulations show that GeV positron jets show
high density (8*10^21/cm^3), attosecond duration (400 as) and a divergence
angle of 14{\deg}. Additionally, ultrabright (2*10^25
photons/s/mm2/mrad2/0.1%BW) collimated attosecond (370 as) {\gamma}-ray flashes
with a laser energy conversion efficiency of 5.6% are emitted. Once realized in
experiment, it may open up new possibilities for a wide variety of
applications.
