In your place I would be dreadfully hurt too.
There may be a good reason why they are not inviting anyone except their parents and siblings, but you do not know it, and neither do I.
Whatever the reason, it would have been kinder of your grand-daughter to tell you what it is, as nicely as possible.
All you can do is try to be glad that she is getting married, and that they are not, like so many these days, using an amount of money they could buy a house for (quite literally) to hold a Hollywood style wedding!
My eldest niece did precisely that, sent us an invitation and then said that we could not stay either with her or her sister or brother as they had "others " staying with them. This meant a hotel room for two nights over and above the cost of a journey from one end of the country to another.
We had accepted the invitation, so we found a cheap hotel, bought one of the few moderately priced items on her wishing-list and attended the ceremony.
Before the reception had really got going the bride swept across to my husband and said, very rudely, "Will you pipe down!" He and one or two of the more rumbustious of the bridegroom's relatives had been chaffing each other good-naturedly. Nothing had been said that anyone had taken offence at, and we, and others who heard what she actually said were flabbergasted.
My husband chose to leave the party then and there, but insisted I stayed.
I am not hinting that your grand-daughter would do anything similar, just mentioning this unpleasant incident to show that you are not alone in wondering what goes on in the heads of the younger generation.
As you can imagine, in hindsight, we would much have preferred not to have been invited than to be treated as we were.
I hope your husband will feel able to add his good wishes to yours when you send a greeting card or a gift. For the sake of a future relationship with your grand-daughter and her husband, it is doubtless better you attend the reception if you both can pretend that you were not hurt by not being invited to the actual wedding.
If your husband is still dreadfully sore about it at that time, which I fully understand he may well be, perhaps he should come down with some diplomatic illness, such as a violent tummy upset, or toothache, leaving you to attend alone.