Antiferromagnetic topological insulators have attracted great attention in the condensed matter physics owing to the fundamental interest in exotic quantum states and topological antiferromagnetic spintronics. Starting with the typical topological insulator of Bi2Te3, we introduced the magnetic order by substituting Gd at the Bi site and tuned the Fermi level by substituting Se at the Te site. That is, we prepared single crystals of Gd xBi2− xTe3− ySe y with various x (= 0.02 and 0.06) and y (= 0.1, 0.2, 0.5, 0.7, 1.0, and 1.5). The magnetic data revealed an antiferromagnetic order for x = 0.06, and the transport data manifested the charge neutral point at y = 0.7. Combining all these results together, the material with x = 0.06 and y = 0.7 is characterized as an antiferromagnetic topological insulator, where we observed exotic magnetotransport properties such as weak antilocalization and negative longitudinal magnetoresistance that are frequently analyzed as chiral anomalies in Weyl materials.