Since you need to qualify a procedure, B2.1 would be a good place to start. I believe AWS sells a GTAW < 1/8" that you might be able to use for structural tube work, but not if you're doing TYK connections.
D1.1 sub clause 1.2 limitations (2) "When base metals thinner than 1/8 in. (3mm) thick are to be welded, the requirements of AWS D1.3, Structural Welding Code - Sheet Steel, should apply. When used in conjunction with AWS 1.3, conformance with the applicable provisions of this code shall be required.