Thanks for the responses everyone!
I would try any addresses you could give me...I just sent my GW letter to the address on an old bill. I had absolutely NO luck on the phone with them...even got to a supervisor. This is my biggest baddie, too! I paid them and two weeks later it showed up on my CR and I lost 45 points! They said they hit the CR's a week BEFORE(delay in the CR's reporting..they even sent me a screen shot of the entry with the date I paid them and the date they reported), and tough luck was all they had to say.
Hoping a GW letter will hit some Rep who just paid dearly at the gas pump and grocery store and might take pity on me!!