If you mean by puddle welding using an oxyfuel gas welding process, the answer is yes. For 12 gage steel sheet, it should not be a problem.
However, why don't you use the GMAW or MIG process for attaching the 12 gage steel sheet? It is faster, cleaner and would probably cost less regarding consumables.
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.