Through the two-dimensional particle-in-cell simulations method, we investigate the generation of γ rays in the interaction between ultraintensity lasers and gas-solid composite targets. The results show that, the laser wakefield are derived by the laser, the electrons are trapped and accelerated to high energy. The laser reflected by the solid target and interacts with the high-energy electrons. The electrons continue to propagate forward, pass through the solid target, then interacts with another incident laser propagating to the left. Consequently, two groups of γ rays with different lengths, energies and delays are generated. The effects of driving laser polarization and pre-plasma density on γ rays are analyzed, and the physical mechanism is discussed. The results show that more γ photons can be generated under the higher density pre-plasma and circularly polarized laser, and the resulting γ rays have the advantages of small divergence angle and high flux.