I'm so very sorry Queenie. We are estranged from our youngest son and only GC so I know only too well that physical pain and being unable to breath.
Even if there was a part of you that thought one day your D would have a child of her own, and you would get the news from a 'stranger', the reality is shocking.
I don't know if during the last 4 years you've tried contacting your D, and if so when the last time that was. If you haven't done so, and I can understand why, I would advise against contacting her in anyway as no doubt it will be seen as you only being interested in your GC.
Your mother's influence over your D is extremely strong, it must be for you to have been estranged in the first place, so when you say you're "not brave enough to step into the arena with (your) mother ever again" be aware that your mother may not be the only person in the arena you'd face; your D could well be there too.
As difficult as I know this is, my advice for what it's worth is to do nothing. Unless the relationship between your D and your mother has changed, your estrangement is unlikely to change.
You D, now a mother herself may come to regret losing her relationship with you, her own mother, and contact you.
As Oopsadaisy has said, there is a choice to be made here but I don't think it's yours, to choose between your mother and your D, I think it's your D who must choose, between her GM and her mother.
Take care and come back and let us know how you're doing
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