I've tried Simple Green and the scrub brush method. They will remove dirt, but not discoloration. The improvement I saw was so slight that I won't repeat the process unless it's a relatively new tire with a grease mark.
Chombi hit it right, many gumwalls are just plain dried out or drying out and no amount of cleaner will change that. If it's a newer tire with no evidence of drying, then go for it.
For drying or dried out tires, I'd like to hear if anyone has a magic fountain of youth for gumwalls.