For anyone else who might stumble across this, I found the solution here:
So in your case, that should be:
import java.awt.Robot
import java.awt.event.KeyEvent
import com.kms.katalon.core.webui.keyword.WebUiBuiltInKeywords as WebUI
WebUI.openBrowser("www.google.com")
Robot robot = new Robot()
robot.keyPress(KeyEvent.VK_META)
robot.keyPress(KeyEvent.VK_F)
robot.keyRelease(KeyEvent.VK_META)
robot.keyRelease(KeyEvent.VK_F)