The interaction of shock-induced ejecta with gas beyond the free surface is a critical unsolved issue and being investigated broadly. Using models containing micrometer-sized gas environments, we perform molecular dynamics simulations to investigate the coupling interactions of surrounding gases with ejecta from shock-loaded tin surface. Ejected microjets experience progressively aggravated deceleration with increasing gas density, and particle flows ahead of jet tips are suppressed. Despite the drag effect, the primary fragmentation process is yet intrinsically dominated by a velocity gradient. The continuous interaction between ejecta and gas leads to the progressive formation of transmitted shock waves in background gases, which is jointly determined by ejecta velocity and thermophysical properties of gas. Meanwhile, a mixing layer between ejecta and gas is directly observed, leading to discrepant mass distributions of ejecta along shock direction. With increasing gas density, the volume density tends to rise in the mixing zone while the zone thickness decreases. Further, with the presence of gases, the size distribution of ejected particles is altered with an outstanding feature of enhanced formation of atomic particles. It is found that the stripping effect of gas dominates the growth of ejecta clusters in the transport process. The stripped particles strongly couple and flow with compressed gas, accompanied by recombination into subsequent clusters. As the gas density increases, both formation and annihilation of atomic particles are promoted. The revealed peculiarities provide microscopic views of ejecta interaction with ambient gas, which would further the understanding of gas effects on the breakup and transport of ejected particles.