It's perfectly Christian for you to mourn and miss your godly father/grandfather, and you just go ahead and mourn him all you need; and
Never underestimate what a wonderful balm it is to your soul that you know that your loved one's Redeemer lives, that He has conquered sin and death, and that your father/grandfather wouldn't leave His presence now for all the riches in the universe.
Dan
That's because he already has all the riches in the universe.