I'm design a trackpad now. But I don't konw how to do with the GND layer, just left it as a blank plane or place hatch fill on it ?
There is a remote control reference design kit which contains a trackpad. Will probably help you to look into the design-files or get hands on the kit.
Check this capsense design guide. http://www.cypress.com/?docID=50746 and this document http://www.cypress.com/?docID=50002. Should give your answers.