If you are using AWS D1.3, max thickness is 0.15 in, so 12 GA is in compliance...ref: Sec 1.5.4
Make sure the WPS has been qualified by testing (Sec 4.2---Engineer's approval), as this joint does not conform to prequalified joints shown in Sec 3--Prequalification of WPSs.
D1.3 Sec 1.3.1 covers approved welding processes...Table 4.1, item 4.4 covers test assembly used for WPS qualification....and Table 4.2, item 4.4 deals with welder qualification requirements. The special inspector must verify WPS and welder's certs in the field.