Haven't read all the posts so sorry if this has already been said... You and dh love to travel? Great! Then you won't mind so much if you have to travel to see dd and family. Also, perhaps, sometimes, they will come and stay with you a few days if that would work. Maybe even just the gc sometimes.
Also, please don't forget there are other ways to keep in touch in between visits - Skype, facetime, cards, letters, etc.
I know this must have come as a shock to you and that you will miss dd and her kids very much. I'm so very sorry. But since ds and family live close by, perhaps it will help to think of it as having the best of both worlds. You get to be the nearby gps, who often see ds and family and maybe are called on to childmind sometimes, etc. But you also get to be those long distance gps, the "special treat," who send fun cards and gifts and come to visit from time to time.