There are conflicting opinions on this, to put it mildly. Placing it in an oven on the self-clean cycle has been suggested, but I don't trust that method since after picking up a GS roaster yesterday, I noticed that the bottom of it was rather thin and warped. I had to flatten out the bottom of the roaster so it wouldn't wobble, and that was enough to convince me that the SCO method wasn't going to be viable. To wit: one member mentioned using the self-clean cycle for aluminum waffle irons and they warped.
Brillo or SOS pads were another suggestion. I don't have any of those on hand, so can't yet report on their effectiveness. The exterior of my roaster is in about the same shape as on your piece. I tested some areas with Bar Keeper's Friend and Totally Awesome spray cleaner and results were less than optimal.
When you search on line you'll find repeated appearances of the old tried and true boiling water and cream of tartar method. That may work fine for the interior surfaces, but logistics for the exterior aren't workable. I don't think there's anything to fix the pitting. Apparently the aluminum cookware scares of the past that claimed pitting was particularly dangerous were unfounded, so I wouldn't worry much about them.
I'll be interested in what you try and how it performed. My roaster is still a work in progress.
By the way, $6 was a real steal for that piece. I paid $20 for my roaster and found the same type on line for a lot more -- one super grimy one was going for $75.
