Disclosed are a thin film transistor (TFT) including an oxide semiconductor layer capable of being applied to high-resolution flat panel display devices requiring high-speed driving, a gate driver including the TFT, and a display device including the gate driver. The TFT includes first oxide semiconductor layer consisting of indium-gallium-zinc-tin oxide (IGZTO) and a second oxide semiconductor layer including indium-gallium-zinc oxide (IGZO). A content ratio (Ga/In) of gallium (Ga) to indium (In) of the second oxide semiconductor layer is higher than a content (Ga/In) of Ga to In of the first oxide semiconductor layer, and a content ratio (Zn/In) of zinc (Zn) to In of the second oxide semiconductor layer is higher than a content (Zn/In) of Zn to In of the first oxide semiconductor layer.