I, too, can see and relate completely to where you are coming from. As noted by many others, it wasn't that your children didn't get a gift but that their grandfather didn't seem to express any thoughtfulness toward them at such a special time for them.
We have one grandmother who has been put in a nursing home and is suffering dementia and my children totally understand that she is not at all able to celebrate Christmas with them like she used to and they appreciate just being with her for a little while that day.
However, my father, (who is still running 5K races at 65 years old!) is often guilty of neglecting them out of what I consider to be a general "obliviousness." There's no reason or excuse and I totally understand and appreciate that it's not intentional but the carelessness does hurt sometimes. Our family is also still dealing with the loss of my mom 5 years ago and my father's remarriage and that adds a considerable amount of emotion to the whole situation (that there just isn't enough room on the DIS to even begin to discuss!

). I finally confronted him when he had promised to attend a soccer game for my DS (something he'd never done in 3 years of him playing) and he decided to do something else at the last minute . . . I told him it hurt that I have to beg him to do something that Mom would have done ten times already without even being asked. And, as noted by many, my son got over it in a heartbeat and I of course am still seething!
I still maintain a relationship with him though and try as much as I can to foster one between him and my children and I would advise you do the same though I realize it's never the same. Thoughts are with you!