I agree with Bathsheba. Yes, it was your son's responsibility to get his own father a card, but we all know that some men simply don't think about these things, and would it really have hurt your DiL to simply remind him to get one, or even buy two when she buys her own father's?! Especially when you were all going to be together and she knew GS was going to give her father a card ... how insensitive/spiteful is that?! Not something I could ever do.
I totally understand the OPs upset, as my son didn't bother to get a card for his father either ... in fact, he forgot his birthday too, but that's another story! And I didn't get a mother's day card either! My DD, on the other hand, never forgets anything, and has spent years reminding her brother of special dates, but gave up once he got married and told our DiL that it was up to her to remind him! She even sent them a calendar with all the special dates printed on it! I think I find it more upsetting as it's something I've never forgotten to do, since I was a small little child. And both my son and daughter were brought up doing the same. Since my son was married though, it all seems to be unimportant, although DiLs parents are still remembered, by both of them!
My own in-laws would probably never have had a card/gift if it wasn't for me, as I was always the one to remind my husband, so I fail to see why other DiLs can't do the same.
Personally, my own conscience wouldn't allow me to be so insensitive or spiteful. I often wonder how my son and DiL would feel if they didn't get mother's day/father's day cards from our GC, or we didn't bother sending them birthday cards/gifts ...? But, of course, I'd never not do it.