The usual method of applying a gold wash (or any color wash) to fondant and gumpaste involves using vodka or another liquid that's high in alcohol, like lemon extract. The reason for this is to prevent the liquid in the paint from melting the sugar in the fondant, since the alcohol will evaporate quickly and there won't be as much liquid sitting on the surface of the cake.
Some people prefer not to use alcohol, though, for health or religious reasons, so they're kind of stuck. However, you can use water to make food coloring paint if you're careful about the amount of water you're using.
Here's a video demonstration:
You can also find ready-to-use metallic food colorings like the Rainbow Dust brand that don't have alcohol listed as an ingredient. Just make sure that whatever you're buying is rated as edible if you're going to apply it to fondant that will be eaten. For gumpaste that will be removed from the cake, it isn't as much of an issue and non-toxic colors are fine.