I don't mean this at all unkindly, but I am amazed that the huge majority seem to feel that it is hurtful for the OP & her husband not to be asked to their granddaughter's choice of a small, intimate wedding.
I suppose it must mean that most of you had extremely close relationships with your DGC. (If not, why on earth would you expect to be invited?)
I scarcely knew one set of grandparents and although fond of the other set wouldn't have imagined for a minute that they'd expect to be invited to a wedding which I'd restricted to my immediate family of parents, siblings and my children.
(Unless, of course DGD has been in the habit of regular visits and getting childcare from her DGF & wife. Then it'd stink that they weren't invited.)
As to the fact that the message was relayed by text not phone call, for most people below, say, 50, that's just how it's done. It almost certainly doesn't mean that DGD didn't care enough to call: she just won't have considered it.
My feeling, with the best will in the world, is that the Grandfather feels so heavily invested in this wedding because of his late son, rather than for his affection for his DGD. But he hasn't any option really other than to accept his son is, sadly, gone, and it's unfair to make his DGD a fill-in for him.