When I got my GMG it was preburned and to season it I brushed the insides with some vegetable oil and cooked up a few pork butts. I have used it most days for the past several months and have done nothing further. I do cook ribs, brisket, beef roasts, butts, pizza, burgers, etc., but most of the cooking is at the lower (220-240) temps. There is a nice coating on the inside but nothing too thick or gooey or too thin where I'm worried about rust. I'd do the brushing to remove the surface rust, then the oil and cook a few butts or bacon with some fat in them. Then just monitor it.
Blues, BBQ and Homebrew!