Bandgap engineering is a process that aims at acquiring desired bandgap from semiconductors for various applications. This is typically done by controlling the structure and composition of the semiconductors. Forming alloys is a powerful technique in achieving continuous bandgap tunability in a wide spectral range. However, this is often limited by the difficulty to synthesize full composition alloys as well as the band bowing effect. Here we show that full composition GaSe1-xTex nanostructures can be synthesized on GaAs (111) substrates by physical vapor transport. As Te content increases, the nanostructures transform from isotropic hexagonal phase to anisotropic monoclinic phase, with both phases coexist at a certain Te content range. Such phase transition causes an anomalous band bowing behavior and gives a large emission wavelength tunability. Our results demonstrate the synthesis of semiconductor alloys far from equilibrium conditions and open up opportunities for bandgap engineering through the phase engineering approach.