Thanks for writing. It sounds like powdery mildew, a very common disease. Help prevent powdery mildew by making sure your plants have good air circulation (don't plant them too closely together) and that the leaves stay dry (so try not to water them with a sprinkler).
If that doesn't help or it's not possible, you can try spraying your gerbera daisies with a fungicide labeled for use on powdery mildew.
---Justin, Senior Garden Editor, BHG.com