I was surprised to find that the credit card field built into Gravity Forms is extraordinarily hard to style, for two reasons. First, there are inline styles - with the !important declaration, no less - hard coded into some of the input elements. Second, there is no class name on the expiration date container or on the individual month/year select elements within it.
Here is that script: http://pastebin.com/vmBLKgWu
I'm pasting this here because I really hope that I'm missing something, and that there is a better way to do what I'm attempting. Please let me know if this is not the case.