This situation is so emotionally charged and complicated, that I highly recommend you seek the guidance of a professional, licensed, trained Clinical Psychologist ASAP.
And that you do NOT take advice from complete strangers on how to manage your relationships with your adult children and grand baby you are now estranged from.
Most people here are very confused why you waited so long to speak up and speak your mind and to offer to be 'that family'. Your silence for so long seems very unhealthy and shows you were ambivalent about making such a huge commitment.....sort of like your very own daughter, very unsure about making a commitment to her very own baby.....that is one strange cycle.
I'm not sure I agree that Grandma and Grandpa are the best candidates to raise the baby any longer, due to the profound lack of direct and honest and timely communication.
What were you waiting for ? To be politely asked? Were you waiting to see if the baby was healthy and you could handle the responsibility? No one knows but you why you waited in silence. And I find that to be alarming considering what your DD was contemplating.
About the best thing you can offer now, is that if for some unforeseen circumstances the relationships with the adoptive family go awry, that you would always be there for them. But that should have been offered so long ago.
I'm sad with you as a mother and a woman that you refrained from emotionally supporting your daughter during this decision. I would imagine your silence was heart breaking.