Gifts for grandkids

No. I would just send them a card. They don’t deserve more if they can’t thank you or remember your birthday.

I stopped when they were 18.
Like you, I got no thank you's, not even an e-card, nothing for my birthdays or Christmas, and very few visits unless they wanted to 'borrow' something - which was never returned.
I know some people stopped when the kids got married or set up house together, and others when the first baby arrived (because the baby 'inherited' the gift giving slot). Perhaps something along those lines?

My grandchildren aren't yet adult but my mother-in-law still gifts her grandchildren (my adult boys/girls).

She used to buy them a pair of gloves, a "fancy" notebook, a pen or a bottle of wine or book tokens for birthdays but the last few years she has "pushed the boat out" and has made quite large money gifts (also to their wives/husbands so there's 10 of them now). £200 for those reaching 30 or 40 but otherwise £30/40. I think she has suddenly realised she wants to gift some of her "rainy day" money now she's nearly 100 - but hasn't actually said that... we just sense it from other things she's doing.

I think I'd still give at least "token" gifts to my grandchildren as adults if I had the finances to do so.
A bottle of wine or a book maybe... or something I knew they'd enjoy eating - speciality coffee, olives etc.

I hope I have a good relationship with them as adults. 🧡

Should just say - my adult children all keep in touch with my mother-in-law - two of them visit her once or twice a month, one visits only occasionally (no excuse here except very disorganised) and one whenever in the country. The youngest pops in from work now and again as he works just a few miles from her. I think she really likes this as the visits are ad-hoc and not too long.

All remember birthdays and do thank-yous - though apparently not very quickly - not quickly enough anyway 🤣

My oldest DGD is now, herself, a mum and deserves all the love and care in the world which I would give her if I could. I do give her gifts of tokens which she can spend on herself. If I gave her cash, it might go into the family kitty. The other DGCs are just emerging into adult-hood. Their parents are good at recommending items that would be welcome such as sporting gear, gaming things or vouchers for cinemas and so on. I'd never let any one of them feel left out.