Think this young couple should be congratulated in not falling into the trap of spending enormous amounts of money on their wedding.
Somewhere they have to draw lines as to who to attend, in this case no g.parents. Cannot see what is wrong with that - particularly as they are still talking about a party later on.
How rude NOT to reply to that text!!!! Perhaps the OP wants to have no further relationship with this g.daughter.
Due to my brothers extremely bad behavior my parents were kept apart from his three children for several years whilst they lived with other g.parents. When their eldest g.child got married, I saw the notice in a newspaper and drove them over to sit inthe public gallery as theyw atched their g.daughter walk up that aisle. So worthwhile, as soon as the couple got back from honeymoon, they came to see the g.parents and were able to have an excellent relationship again from then onwards.
My son and his finacee were actually living in my house, when they secretly got married, not telling me or anyone else. Just had two witnesses. Due to a family tragedy a couple of weeks later, they never got round to telling us. It was a year before we all found out - did it matter??? Not at all. Relationships matter som uch more than any particular occasion.
Send a lovely card to this couple on the day, and a nice pressie and look forward to the party.
Stop being selfish - it is THEIR weddding, it is not about you.