Now your granddaughter will leave this loving environment you created for her and travel to a new adventure with her mom and step-dad. A move far away should not sentence your granddaughter to a life of emotional insecurity. Framing the move in the most favorable manner possible is a must. Be gratified and take great comfort in knowing that you have raised a delightful, secure, loved, little girl. I hope your daughter and her husband have learned much about childrearing from your example.
Stay in touch with your granddaughter as much as you can - cards, calls, visits when possible. Don't let her pick up on any fear for her well-being you may have. This will worry her and make her even more disturbed about moving.
I know you'll find ways to continue to be a major, loving influence in her life. You certainly put the parent in grandparent. You gave her, and will continue to give her, the most precious gift she could receive- your unconditional love and care. Good luck to you all.