Oddly enough, I just cleaned some Gottlieb score reels last night as well. I used windex on a q-tip and worked my way around the numbers. It's very tedious and you have to be extremely careful because if you hit the numbers, with any pressure, the ink will come off. I'm using very, very little windex on the q-tip, just enough to moisten the tip and loosen the dirt.
As many others have said, the numbers on the gottlieb reels are very fragile. Whatever method you use, you must be careful. The next time I have to clean gottlieb reels, I'm going to try the white bread method.