When welding to the requirements of AWS D1.1, the 6GR test is required to qualify the welding procedure and the welder that will weld T, K, and Y joints without backing. Since the root is already welded, the welder depositing any subsequent weld metal is welding with backing. Table 4.10 (D1.1:2006) for welder qualification has a footnote that allows the welder welding with backing to be tested using the 6G test with or without backing.
Best regards - Al