FAMILY IS ALWAYS FIRST as i am sure you know John .BUT! I am sure your daughter knows how much this Gig means to Dad and she would of course want you there when the baby is born but you must remember her Husband is her Life now even though she loves you Dad to death.
First i would concider this!1. Can i set the gig up so i can leave if she really needs me ?
2. How far away am i going to be from her?
3. What are the chances of her having the baby the same night i have my gig?
John i guess i would talk to her and ask her to give me her honest feelings about Ol Dad being there at the moment of birth.
If she really wants me to be there I would be there Gig or No Gig!
But she may surprise you with an answer that she now has Hubby to be there at birth and Ol Dad can come right after the Gig!
As hard as it would be for me to talk to her i would really think about asking her what she wants me to do. Once you have her answer what else can you do but honor your daughters feelings and if so there will always be another Gig maybe even better!!
My 2 cents .S.Dee