You can use pretty much anything you want. Even an abrasive cleaner(with a ScotchBrite sponge (for exampled). All Purpose glass cleaner works well, and the Mr. Clean "Magic" sponge, well is magic.
While the G6 card will tend to absorb dirt and need to be cleaned abrasively, the G7 card will not absorb dirt, and should be cleaned with a non-abrasive cleaner. Again All Purpose glass cleaner works well, Any dirt is right on the surface and should come right off. If there is a tough situation like someone marking the card with a Sharpie, I have been able to get that off with the more abrasive treatments, but that should not be the normal method.
Both cards are waterproof, so the cleaning is really easy in that they can be cleaned in the sink, and/or rinsed off at any time.